sqlalchemy.dialects.mysql.base

.. dialect:: mysql :name: MySQL / MariaDB :full_support: 5.6, 5.7, 8.0 / 10.4, 10.5 :normal_support: 5.6+ / 10+ :best_effort: 5.0.2+ / 5.0.2+ Supported Versions and Features ------------------------------- SQLAlchemy supports MySQL starting with version 5.0.2 through modern releases, as well as all modern versions of MariaDB. See the official MySQL documentation for detailed information about features supported in any given server release. .. versionchanged:: 1.4 minimum MySQL version supported is now 5.0.2. MariaDB Support ~~~~~~~~~~~~~~~ The MariaDB variant of MySQL retains fundamental compatibility with MySQL's protocols however the development of these two products continues to diverge. Within the realm of SQLAlchemy, the two databases have a small number of syntactical and behavioral differences that SQLAlchemy accommodates automatically. To connect to a MariaDB database, no changes to the database URL are required:: engine = create_engine("mysql+pymysql://user:pass@some_mariadb/dbname?charset=utf8mb4") Upon first connect, the SQLAlchemy dialect employs a server version detection scheme that determines if the backing database reports as MariaDB. Based on this flag, the dialect can make different choices in those of areas where its behavior must be different. .. _mysql_mariadb_only_mode: MariaDB-Only Mode ~~~~~~~~~~~~~~~~~ The dialect also supports an **optional** "MariaDB-only" mode of connection, which may be useful for the case where an application makes use of MariaDB-specific features and is not compatible with a MySQL database. To use this mode of operation, replace the "mysql" token in the above URL with "mariadb":: engine = create_engine("mariadb+pymysql://user:pass@some_mariadb/dbname?charset=utf8mb4") The above engine, upon first connect, will raise an error if the server version detection detects that the backing database is not MariaDB. When using an engine with ``"mariadb"`` as the dialect name, **all mysql-specific options that include the name "mysql" in them are now named with "mariadb"**. This means options like ``mysql_engine`` should be named ``mariadb_engine``, etc. Both "mysql" and "mariadb" options can be used simultaneously for applications that use URLs with both "mysql" and "mariadb" dialects:: my_table = Table( "mytable", metadata, Column("id", Integer, primary_key=True), Column("textdata", String(50)), mariadb_engine="InnoDB", mysql_engine="InnoDB", ) Index( "textdata_ix", my_table.c.textdata, mysql_prefix="FULLTEXT", mariadb_prefix="FULLTEXT", ) Similar behavior will occur when the above structures are reflected, i.e. the "mariadb" prefix will be present in the option names when the database URL is based on the "mariadb" name. .. versionadded:: 1.4 Added "mariadb" dialect name supporting "MariaDB-only mode" for the MySQL dialect. .. _mysql_connection_timeouts: Connection Timeouts and Disconnects ----------------------------------- MySQL / MariaDB feature an automatic connection close behavior, for connections that have been idle for a fixed period of time, defaulting to eight hours. To circumvent having this issue, use the :paramref:`_sa.create_engine.pool_recycle` option which ensures that a connection will be discarded and replaced with a new one if it has been present in the pool for a fixed number of seconds:: engine = create_engine('mysql+mysqldb://...', pool_recycle=3600) For more comprehensive disconnect detection of pooled connections, including accommodation of server restarts and network issues, a pre-ping approach may be employed. See :ref:`pool_disconnects` for current approaches. .. seealso:: :ref:`pool_disconnects` - Background on several techniques for dealing with timed out connections as well as database restarts. .. _mysql_storage_engines: CREATE TABLE arguments including Storage Engines ------------------------------------------------ Both MySQL's and MariaDB's CREATE TABLE syntax includes a wide array of special options, including ``ENGINE``, ``CHARSET``, ``MAX_ROWS``, ``ROW_FORMAT``, ``INSERT_METHOD``, and many more. To accommodate the rendering of these arguments, specify the form ``mysql_argument_name="value"``. For example, to specify a table with ``ENGINE`` of ``InnoDB``, ``CHARSET`` of ``utf8mb4``, and ``KEY_BLOCK_SIZE`` of ``1024``:: Table('mytable', metadata, Column('data', String(32)), mysql_engine='InnoDB', mysql_charset='utf8mb4', mysql_key_block_size="1024" ) When supporting :ref:`mysql_mariadb_only_mode` mode, similar keys against the "mariadb" prefix must be included as well. The values can of course vary independently so that different settings on MySQL vs. MariaDB may be maintained:: # support both "mysql" and "mariadb-only" engine URLs Table('mytable', metadata, Column('data', String(32)), mysql_engine='InnoDB', mariadb_engine='InnoDB', mysql_charset='utf8mb4', mariadb_charset='utf8', mysql_key_block_size="1024" mariadb_key_block_size="1024" ) The MySQL / MariaDB dialects will normally transfer any keyword specified as ``mysql_keyword_name`` to be rendered as ``KEYWORD_NAME`` in the ``CREATE TABLE`` statement. A handful of these names will render with a space instead of an underscore; to support this, the MySQL dialect has awareness of these particular names, which include ``DATA DIRECTORY`` (e.g. ``mysql_data_directory``), ``CHARACTER SET`` (e.g. ``mysql_character_set``) and ``INDEX DIRECTORY`` (e.g. ``mysql_index_directory``). The most common argument is ``mysql_engine``, which refers to the storage engine for the table. Historically, MySQL server installations would default to ``MyISAM`` for this value, although newer versions may be defaulting to ``InnoDB``. The ``InnoDB`` engine is typically preferred for its support of transactions and foreign keys. A :class:`_schema.Table` that is created in a MySQL / MariaDB database with a storage engine of ``MyISAM`` will be essentially non-transactional, meaning any INSERT/UPDATE/DELETE statement referring to this table will be invoked as autocommit. It also will have no support for foreign key constraints; while the ``CREATE TABLE`` statement accepts foreign key options, when using the ``MyISAM`` storage engine these arguments are discarded. Reflecting such a table will also produce no foreign key constraint information. For fully atomic transactions as well as support for foreign key constraints, all participating ``CREATE TABLE`` statements must specify a transactional engine, which in the vast majority of cases is ``InnoDB``. Case Sensitivity and Table Reflection ------------------------------------- Both MySQL and MariaDB have inconsistent support for case-sensitive identifier names, basing support on specific details of the underlying operating system. However, it has been observed that no matter what case sensitivity behavior is present, the names of tables in foreign key declarations are *always* received from the database as all-lower case, making it impossible to accurately reflect a schema where inter-related tables use mixed-case identifier names. Therefore it is strongly advised that table names be declared as all lower case both within SQLAlchemy as well as on the MySQL / MariaDB database itself, especially if database reflection features are to be used. .. _mysql_isolation_level: Transaction Isolation Level --------------------------- All MySQL / MariaDB dialects support setting of transaction isolation level both via a dialect-specific parameter :paramref:`_sa.create_engine.isolation_level` accepted by :func:`_sa.create_engine`, as well as the :paramref:`.Connection.execution_options.isolation_level` argument as passed to :meth:`_engine.Connection.execution_options`. This feature works by issuing the command ``SET SESSION TRANSACTION ISOLATION LEVEL <level>`` for each new connection. For the special AUTOCOMMIT isolation level, DBAPI-specific techniques are used. To set isolation level using :func:`_sa.create_engine`:: engine = create_engine( "mysql+mysqldb://scott:tiger@localhost/test", isolation_level="READ UNCOMMITTED" ) To set using per-connection execution options:: connection = engine.connect() connection = connection.execution_options( isolation_level="READ COMMITTED" ) Valid values for ``isolation_level`` include: * ``READ COMMITTED`` * ``READ UNCOMMITTED`` * ``REPEATABLE READ`` * ``SERIALIZABLE`` * ``AUTOCOMMIT`` The special ``AUTOCOMMIT`` value makes use of the various "autocommit" attributes provided by specific DBAPIs, and is currently supported by MySQLdb, MySQL-Client, MySQL-Connector Python, and PyMySQL. Using it, the database connection will return true for the value of ``SELECT @@autocommit;``. There are also more options for isolation level configurations, such as "sub-engine" objects linked to a main :class:`_engine.Engine` which each apply different isolation level settings. See the discussion at :ref:`dbapi_autocommit` for background. .. seealso:: :ref:`dbapi_autocommit` AUTO_INCREMENT Behavior ----------------------- When creating tables, SQLAlchemy will automatically set ``AUTO_INCREMENT`` on the first :class:`.Integer` primary key column which is not marked as a foreign key:: >>> t = Table('mytable', metadata, ... Column('mytable_id', Integer, primary_key=True) ... ) >>> t.create() CREATE TABLE mytable ( id INTEGER NOT NULL AUTO_INCREMENT, PRIMARY KEY (id) ) You can disable this behavior by passing ``False`` to the :paramref:`_schema.Column.autoincrement` argument of :class:`_schema.Column`. This flag can also be used to enable auto-increment on a secondary column in a multi-column key for some storage engines:: Table('mytable', metadata, Column('gid', Integer, primary_key=True, autoincrement=False), Column('id', Integer, primary_key=True) ) .. _mysql_ss_cursors: Server Side Cursors ------------------- Server-side cursor support is available for the mysqlclient, PyMySQL, mariadbconnector dialects and may also be available in others. This makes use of either the "buffered=True/False" flag if available or by using a class such as ``MySQLdb.cursors.SSCursor`` or ``pymysql.cursors.SSCursor`` internally. Server side cursors are enabled on a per-statement basis by using the :paramref:`.Connection.execution_options.stream_results` connection execution option:: with engine.connect() as conn: result = conn.execution_options(stream_results=True).execute(text("select * from table")) Note that some kinds of SQL statements may not be supported with server side cursors; generally, only SQL statements that return rows should be used with this option. .. deprecated:: 1.4 The dialect-level server_side_cursors flag is deprecated and will be removed in a future release. Please use the :paramref:`_engine.Connection.stream_results` execution option for unbuffered cursor support. .. seealso:: :ref:`engine_stream_results` .. _mysql_unicode: Unicode ------- Charset Selection ~~~~~~~~~~~~~~~~~ Most MySQL / MariaDB DBAPIs offer the option to set the client character set for a connection. This is typically delivered using the ``charset`` parameter in the URL, such as:: e = create_engine( "mysql+pymysql://scott:tiger@localhost/test?charset=utf8mb4") This charset is the **client character set** for the connection. Some MySQL DBAPIs will default this to a value such as ``latin1``, and some will make use of the ``default-character-set`` setting in the ``my.cnf`` file as well. Documentation for the DBAPI in use should be consulted for specific behavior. The encoding used for Unicode has traditionally been ``'utf8'``. However, for MySQL versions 5.5.3 and MariaDB 5.5 on forward, a new MySQL-specific encoding ``'utf8mb4'`` has been introduced, and as of MySQL 8.0 a warning is emitted by the server if plain ``utf8`` is specified within any server-side directives, replaced with ``utf8mb3``. The rationale for this new encoding is due to the fact that MySQL's legacy utf-8 encoding only supports codepoints up to three bytes instead of four. Therefore, when communicating with a MySQL or MariaDB database that includes codepoints more than three bytes in size, this new charset is preferred, if supported by both the database as well as the client DBAPI, as in:: e = create_engine( "mysql+pymysql://scott:tiger@localhost/test?charset=utf8mb4") All modern DBAPIs should support the ``utf8mb4`` charset. In order to use ``utf8mb4`` encoding for a schema that was created with legacy ``utf8``, changes to the MySQL/MariaDB schema and/or server configuration may be required. .. seealso:: `The utf8mb4 Character Set \ <https://dev.mysql.com/doc/refman/5.5/en/charset-unicode-utf8mb4.html>`_ - \ in the MySQL documentation .. _mysql_binary_introducer: Dealing with Binary Data Warnings and Unicode ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ MySQL versions 5.6, 5.7 and later (not MariaDB at the time of this writing) now emit a warning when attempting to pass binary data to the database, while a character set encoding is also in place, when the binary data itself is not valid for that encoding:: default.py:509: Warning: (1300, "Invalid utf8mb4 character string: 'F9876A'") cursor.execute(statement, parameters) This warning is due to the fact that the MySQL client library is attempting to interpret the binary string as a unicode object even if a datatype such as :class:`.LargeBinary` is in use. To resolve this, the SQL statement requires a binary "character set introducer" be present before any non-NULL value that renders like this:: INSERT INTO table (data) VALUES (_binary %s) These character set introducers are provided by the DBAPI driver, assuming the use of mysqlclient or PyMySQL (both of which are recommended). Add the query string parameter ``binary_prefix=true`` to the URL to repair this warning:: # mysqlclient engine = create_engine( "mysql+mysqldb://scott:tiger@localhost/test?charset=utf8mb4&binary_prefix=true") # PyMySQL engine = create_engine( "mysql+pymysql://scott:tiger@localhost/test?charset=utf8mb4&binary_prefix=true") The ``binary_prefix`` flag may or may not be supported by other MySQL drivers. SQLAlchemy itself cannot render this ``_binary`` prefix reliably, as it does not work with the NULL value, which is valid to be sent as a bound parameter. As the MySQL driver renders parameters directly into the SQL string, it's the most efficient place for this additional keyword to be passed. .. seealso:: `Character set introducers <https://dev.mysql.com/doc/refman/5.7/en/charset-introducer.html>`_ - on the MySQL website ANSI Quoting Style ------------------ MySQL / MariaDB feature two varieties of identifier "quoting style", one using backticks and the other using quotes, e.g. ```some_identifier``` vs. ``"some_identifier"``. All MySQL dialects detect which version is in use by checking the value of :ref:`sql_mode<mysql_sql_mode>` when a connection is first established with a particular :class:`_engine.Engine`. This quoting style comes into play when rendering table and column names as well as when reflecting existing database structures. The detection is entirely automatic and no special configuration is needed to use either quoting style. .. _mysql_sql_mode: Changing the sql_mode --------------------- MySQL supports operating in multiple `Server SQL Modes <https://dev.mysql.com/doc/refman/8.0/en/sql-mode.html>`_ for both Servers and Clients. To change the ``sql_mode`` for a given application, a developer can leverage SQLAlchemy's Events system. In the following example, the event system is used to set the ``sql_mode`` on the ``first_connect`` and ``connect`` events:: from sqlalchemy import create_engine, event eng = create_engine("mysql+mysqldb://scott:tiger@localhost/test", echo='debug') # `insert=True` will ensure this is the very first listener to run @event.listens_for(eng, "connect", insert=True) def connect(dbapi_connection, connection_record): cursor = dbapi_connection.cursor() cursor.execute("SET sql_mode = 'STRICT_ALL_TABLES'") conn = eng.connect() In the example illustrated above, the "connect" event will invoke the "SET" statement on the connection at the moment a particular DBAPI connection is first created for a given Pool, before the connection is made available to the connection pool. Additionally, because the function was registered with ``insert=True``, it will be prepended to the internal list of registered functions. MySQL / MariaDB SQL Extensions ------------------------------ Many of the MySQL / MariaDB SQL extensions are handled through SQLAlchemy's generic function and operator support:: table.select(table.c.password==func.md5('plaintext')) table.select(table.c.username.op('regexp')('^[a-d]')) And of course any valid SQL statement can be executed as a string as well. Some limited direct support for MySQL / MariaDB extensions to SQL is currently available. * INSERT..ON DUPLICATE KEY UPDATE: See :ref:`mysql_insert_on_duplicate_key_update` * SELECT pragma, use :meth:`_expression.Select.prefix_with` and :meth:`_query.Query.prefix_with`:: select(...).prefix_with(['HIGH_PRIORITY', 'SQL_SMALL_RESULT']) * UPDATE with LIMIT:: update(..., mysql_limit=10, mariadb_limit=10) * optimizer hints, use :meth:`_expression.Select.prefix_with` and :meth:`_query.Query.prefix_with`:: select(...).prefix_with("/*+ NO_RANGE_OPTIMIZATION(t4 PRIMARY) */") * index hints, use :meth:`_expression.Select.with_hint` and :meth:`_query.Query.with_hint`:: select(...).with_hint(some_table, "USE INDEX xyz") * MATCH operator support:: from sqlalchemy.dialects.mysql import match select(...).where(match(col1, col2, against="some expr").in_boolean_mode()) .. seealso:: :class:`_mysql.match` INSERT/DELETE...RETURNING ------------------------- The MariaDB dialect supports 10.5+'s ``INSERT..RETURNING`` and ``DELETE..RETURNING`` (10.0+) syntaxes. ``INSERT..RETURNING`` may be used automatically in some cases in order to fetch newly generated identifiers in place of the traditional approach of using ``cursor.lastrowid``, however ``cursor.lastrowid`` is currently still preferred for simple single-statement cases for its better performance. To specify an explicit ``RETURNING`` clause, use the :meth:`._UpdateBase.returning` method on a per-statement basis:: # INSERT..RETURNING result = connection.execute( table.insert(). values(name='foo'). returning(table.c.col1, table.c.col2) ) print(result.all()) # DELETE..RETURNING result = connection.execute( table.delete(). where(table.c.name=='foo'). returning(table.c.col1, table.c.col2) ) print(result.all()) .. versionadded:: 2.0 Added support for MariaDB RETURNING .. _mysql_insert_on_duplicate_key_update: INSERT...ON DUPLICATE KEY UPDATE (Upsert) ------------------------------------------ MySQL / MariaDB allow "upserts" (update or insert) of rows into a table via the ``ON DUPLICATE KEY UPDATE`` clause of the ``INSERT`` statement. A candidate row will only be inserted if that row does not match an existing primary or unique key in the table; otherwise, an UPDATE will be performed. The statement allows for separate specification of the values to INSERT versus the values for UPDATE. SQLAlchemy provides ``ON DUPLICATE KEY UPDATE`` support via the MySQL-specific :func:`.mysql.insert()` function, which provides the generative method :meth:`~.mysql.Insert.on_duplicate_key_update`: .. sourcecode:: pycon+sql >>> from sqlalchemy.dialects.mysql import insert >>> insert_stmt = insert(my_table).values( ... id='some_existing_id', ... data='inserted value') >>> on_duplicate_key_stmt = insert_stmt.on_duplicate_key_update( ... data=insert_stmt.inserted.data, ... status='U' ... ) >>> print(on_duplicate_key_stmt) {printsql}INSERT INTO my_table (id, data) VALUES (%s, %s) ON DUPLICATE KEY UPDATE data = VALUES(data), status = %s Unlike PostgreSQL's "ON CONFLICT" phrase, the "ON DUPLICATE KEY UPDATE" phrase will always match on any primary key or unique key, and will always perform an UPDATE if there's a match; there are no options for it to raise an error or to skip performing an UPDATE. ``ON DUPLICATE KEY UPDATE`` is used to perform an update of the already existing row, using any combination of new values as well as values from the proposed insertion. These values are normally specified using keyword arguments passed to the :meth:`_mysql.Insert.on_duplicate_key_update` given column key values (usually the name of the column, unless it specifies :paramref:`_schema.Column.key` ) as keys and literal or SQL expressions as values: .. sourcecode:: pycon+sql >>> insert_stmt = insert(my_table).values( ... id='some_existing_id', ... data='inserted value') >>> on_duplicate_key_stmt = insert_stmt.on_duplicate_key_update( ... data="some data", ... updated_at=func.current_timestamp(), ... ) >>> print(on_duplicate_key_stmt) {printsql}INSERT INTO my_table (id, data) VALUES (%s, %s) ON DUPLICATE KEY UPDATE data = %s, updated_at = CURRENT_TIMESTAMP In a manner similar to that of :meth:`.UpdateBase.values`, other parameter forms are accepted, including a single dictionary: .. sourcecode:: pycon+sql >>> on_duplicate_key_stmt = insert_stmt.on_duplicate_key_update( ... {"data": "some data", "updated_at": func.current_timestamp()}, ... ) as well as a list of 2-tuples, which will automatically provide a parameter-ordered UPDATE statement in a manner similar to that described at :ref:`tutorial_parameter_ordered_updates`. Unlike the :class:`_expression.Update` object, no special flag is needed to specify the intent since the argument form is this context is unambiguous: .. sourcecode:: pycon+sql >>> on_duplicate_key_stmt = insert_stmt.on_duplicate_key_update( ... [ ... ("data", "some data"), ... ("updated_at", func.current_timestamp()), ... ] ... ) >>> print(on_duplicate_key_stmt) {printsql}INSERT INTO my_table (id, data) VALUES (%s, %s) ON DUPLICATE KEY UPDATE data = %s, updated_at = CURRENT_TIMESTAMP .. versionchanged:: 1.3 support for parameter-ordered UPDATE clause within MySQL ON DUPLICATE KEY UPDATE .. warning:: The :meth:`_mysql.Insert.on_duplicate_key_update` method does **not** take into account Python-side default UPDATE values or generation functions, e.g. e.g. those specified using :paramref:`_schema.Column.onupdate`. These values will not be exercised for an ON DUPLICATE KEY style of UPDATE, unless they are manually specified explicitly in the parameters. In order to refer to the proposed insertion row, the special alias :attr:`_mysql.Insert.inserted` is available as an attribute on the :class:`_mysql.Insert` object; this object is a :class:`_expression.ColumnCollection` which contains all columns of the target table: .. sourcecode:: pycon+sql >>> stmt = insert(my_table).values( ... id='some_id', ... data='inserted value', ... author='jlh') >>> do_update_stmt = stmt.on_duplicate_key_update( ... data="updated value", ... author=stmt.inserted.author ... ) >>> print(do_update_stmt) {printsql}INSERT INTO my_table (id, data, author) VALUES (%s, %s, %s) ON DUPLICATE KEY UPDATE data = %s, author = VALUES(author) When rendered, the "inserted" namespace will produce the expression ``VALUES(<columnname>)``. .. versionadded:: 1.2 Added support for MySQL ON DUPLICATE KEY UPDATE clause rowcount Support ---------------- SQLAlchemy standardizes the DBAPI ``cursor.rowcount`` attribute to be the usual definition of "number of rows matched by an UPDATE or DELETE" statement. This is in contradiction to the default setting on most MySQL DBAPI drivers, which is "number of rows actually modified/deleted". For this reason, the SQLAlchemy MySQL dialects always add the ``constants.CLIENT.FOUND_ROWS`` flag, or whatever is equivalent for the target dialect, upon connection. This setting is currently hardcoded. .. seealso:: :attr:`_engine.CursorResult.rowcount` .. _mysql_indexes: MySQL / MariaDB- Specific Index Options ----------------------------------------- MySQL and MariaDB-specific extensions to the :class:`.Index` construct are available. Index Length ~~~~~~~~~~~~~ MySQL and MariaDB both provide an option to create index entries with a certain length, where "length" refers to the number of characters or bytes in each value which will become part of the index. SQLAlchemy provides this feature via the ``mysql_length`` and/or ``mariadb_length`` parameters:: Index('my_index', my_table.c.data, mysql_length=10, mariadb_length=10) Index('a_b_idx', my_table.c.a, my_table.c.b, mysql_length={'a': 4, 'b': 9}) Index('a_b_idx', my_table.c.a, my_table.c.b, mariadb_length={'a': 4, 'b': 9}) Prefix lengths are given in characters for nonbinary string types and in bytes for binary string types. The value passed to the keyword argument *must* be either an integer (and, thus, specify the same prefix length value for all columns of the index) or a dict in which keys are column names and values are prefix length values for corresponding columns. MySQL and MariaDB only allow a length for a column of an index if it is for a CHAR, VARCHAR, TEXT, BINARY, VARBINARY and BLOB. Index Prefixes ~~~~~~~~~~~~~~ MySQL storage engines permit you to specify an index prefix when creating an index. SQLAlchemy provides this feature via the ``mysql_prefix`` parameter on :class:`.Index`:: Index('my_index', my_table.c.data, mysql_prefix='FULLTEXT') The value passed to the keyword argument will be simply passed through to the underlying CREATE INDEX, so it *must* be a valid index prefix for your MySQL storage engine. .. versionadded:: 1.1.5 .. seealso:: `CREATE INDEX <https://dev.mysql.com/doc/refman/5.0/en/create-index.html>`_ - MySQL documentation Index Types ~~~~~~~~~~~~~ Some MySQL storage engines permit you to specify an index type when creating an index or primary key constraint. SQLAlchemy provides this feature via the ``mysql_using`` parameter on :class:`.Index`:: Index('my_index', my_table.c.data, mysql_using='hash', mariadb_using='hash') As well as the ``mysql_using`` parameter on :class:`.PrimaryKeyConstraint`:: PrimaryKeyConstraint("data", mysql_using='hash', mariadb_using='hash') The value passed to the keyword argument will be simply passed through to the underlying CREATE INDEX or PRIMARY KEY clause, so it *must* be a valid index type for your MySQL storage engine. More information can be found at: https://dev.mysql.com/doc/refman/5.0/en/create-index.html https://dev.mysql.com/doc/refman/5.0/en/create-table.html Index Parsers ~~~~~~~~~~~~~ CREATE FULLTEXT INDEX in MySQL also supports a "WITH PARSER" option. This is available using the keyword argument ``mysql_with_parser``:: Index( 'my_index', my_table.c.data, mysql_prefix='FULLTEXT', mysql_with_parser="ngram", mariadb_prefix='FULLTEXT', mariadb_with_parser="ngram", ) .. versionadded:: 1.3 .. _mysql_foreign_keys: MySQL / MariaDB Foreign Keys ----------------------------- MySQL and MariaDB's behavior regarding foreign keys has some important caveats. Foreign Key Arguments to Avoid ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Neither MySQL nor MariaDB support the foreign key arguments "DEFERRABLE", "INITIALLY", or "MATCH". Using the ``deferrable`` or ``initially`` keyword argument with :class:`_schema.ForeignKeyConstraint` or :class:`_schema.ForeignKey` will have the effect of these keywords being rendered in a DDL expression, which will then raise an error on MySQL or MariaDB. In order to use these keywords on a foreign key while having them ignored on a MySQL / MariaDB backend, use a custom compile rule:: from sqlalchemy.ext.compiler import compiles from sqlalchemy.schema import ForeignKeyConstraint @compiles(ForeignKeyConstraint, "mysql", "mariadb") def process(element, compiler, **kw): element.deferrable = element.initially = None return compiler.visit_foreign_key_constraint(element, **kw) The "MATCH" keyword is in fact more insidious, and is explicitly disallowed by SQLAlchemy in conjunction with the MySQL or MariaDB backends. This argument is silently ignored by MySQL / MariaDB, but in addition has the effect of ON UPDATE and ON DELETE options also being ignored by the backend. Therefore MATCH should never be used with the MySQL / MariaDB backends; as is the case with DEFERRABLE and INITIALLY, custom compilation rules can be used to correct a ForeignKeyConstraint at DDL definition time. Reflection of Foreign Key Constraints ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Not all MySQL / MariaDB storage engines support foreign keys. When using the very common ``MyISAM`` MySQL storage engine, the information loaded by table reflection will not include foreign keys. For these tables, you may supply a :class:`~sqlalchemy.ForeignKeyConstraint` at reflection time:: Table('mytable', metadata, ForeignKeyConstraint(['other_id'], ['othertable.other_id']), autoload_with=engine ) .. seealso:: :ref:`mysql_storage_engines` .. _mysql_unique_constraints: MySQL / MariaDB Unique Constraints and Reflection ---------------------------------------------------- SQLAlchemy supports both the :class:`.Index` construct with the flag ``unique=True``, indicating a UNIQUE index, as well as the :class:`.UniqueConstraint` construct, representing a UNIQUE constraint. Both objects/syntaxes are supported by MySQL / MariaDB when emitting DDL to create these constraints. However, MySQL / MariaDB does not have a unique constraint construct that is separate from a unique index; that is, the "UNIQUE" constraint on MySQL / MariaDB is equivalent to creating a "UNIQUE INDEX". When reflecting these constructs, the :meth:`_reflection.Inspector.get_indexes` and the :meth:`_reflection.Inspector.get_unique_constraints` methods will **both** return an entry for a UNIQUE index in MySQL / MariaDB. However, when performing full table reflection using ``Table(..., autoload_with=engine)``, the :class:`.UniqueConstraint` construct is **not** part of the fully reflected :class:`_schema.Table` construct under any circumstances; this construct is always represented by a :class:`.Index` with the ``unique=True`` setting present in the :attr:`_schema.Table.indexes` collection. TIMESTAMP / DATETIME issues --------------------------- .. _mysql_timestamp_onupdate: Rendering ON UPDATE CURRENT TIMESTAMP for MySQL / MariaDB's explicit_defaults_for_timestamp ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ MySQL / MariaDB have historically expanded the DDL for the :class:`_types.TIMESTAMP` datatype into the phrase "TIMESTAMP DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP", which includes non-standard SQL that automatically updates the column with the current timestamp when an UPDATE occurs, eliminating the usual need to use a trigger in such a case where server-side update changes are desired. MySQL 5.6 introduced a new flag `explicit_defaults_for_timestamp <https://dev.mysql.com/doc/refman/5.6/en/server-system-variables.html #sysvar_explicit_defaults_for_timestamp>`_ which disables the above behavior, and in MySQL 8 this flag defaults to true, meaning in order to get a MySQL "on update timestamp" without changing this flag, the above DDL must be rendered explicitly. Additionally, the same DDL is valid for use of the ``DATETIME`` datatype as well. SQLAlchemy's MySQL dialect does not yet have an option to generate MySQL's "ON UPDATE CURRENT_TIMESTAMP" clause, noting that this is not a general purpose "ON UPDATE" as there is no such syntax in standard SQL. SQLAlchemy's :paramref:`_schema.Column.server_onupdate` parameter is currently not related to this special MySQL behavior. To generate this DDL, make use of the :paramref:`_schema.Column.server_default` parameter and pass a textual clause that also includes the ON UPDATE clause:: from sqlalchemy import Table, MetaData, Column, Integer, String, TIMESTAMP from sqlalchemy import text metadata = MetaData() mytable = Table( "mytable", metadata, Column('id', Integer, primary_key=True), Column('data', String(50)), Column( 'last_updated', TIMESTAMP, server_default=text("CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP") ) ) The same instructions apply to use of the :class:`_types.DateTime` and :class:`_types.DATETIME` datatypes:: from sqlalchemy import DateTime mytable = Table( "mytable", metadata, Column('id', Integer, primary_key=True), Column('data', String(50)), Column( 'last_updated', DateTime, server_default=text("CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP") ) ) Even though the :paramref:`_schema.Column.server_onupdate` feature does not generate this DDL, it still may be desirable to signal to the ORM that this updated value should be fetched. This syntax looks like the following:: from sqlalchemy.schema import FetchedValue class MyClass(Base): __tablename__ = 'mytable' id = Column(Integer, primary_key=True) data = Column(String(50)) last_updated = Column( TIMESTAMP, server_default=text("CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP"), server_onupdate=FetchedValue() ) .. _mysql_timestamp_null: TIMESTAMP Columns and NULL ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ MySQL historically enforces that a column which specifies the TIMESTAMP datatype implicitly includes a default value of CURRENT_TIMESTAMP, even though this is not stated, and additionally sets the column as NOT NULL, the opposite behavior vs. that of all other datatypes:: mysql> CREATE TABLE ts_test ( -> a INTEGER, -> b INTEGER NOT NULL, -> c TIMESTAMP, -> d TIMESTAMP DEFAULT CURRENT_TIMESTAMP, -> e TIMESTAMP NULL); Query OK, 0 rows affected (0.03 sec) mysql> SHOW CREATE TABLE ts_test; +---------+----------------------------------------------------- | Table | Create Table +---------+----------------------------------------------------- | ts_test | CREATE TABLE `ts_test` ( `a` int(11) DEFAULT NULL, `b` int(11) NOT NULL, `c` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP, `d` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP, `e` timestamp NULL DEFAULT NULL ) ENGINE=MyISAM DEFAULT CHARSET=latin1 Above, we see that an INTEGER column defaults to NULL, unless it is specified with NOT NULL. But when the column is of type TIMESTAMP, an implicit default of CURRENT_TIMESTAMP is generated which also coerces the column to be a NOT NULL, even though we did not specify it as such. This behavior of MySQL can be changed on the MySQL side using the `explicit_defaults_for_timestamp <https://dev.mysql.com/doc/refman/5.6/en/server-system-variables.html #sysvar_explicit_defaults_for_timestamp>`_ configuration flag introduced in MySQL 5.6. With this server setting enabled, TIMESTAMP columns behave like any other datatype on the MySQL side with regards to defaults and nullability. However, to accommodate the vast majority of MySQL databases that do not specify this new flag, SQLAlchemy emits the "NULL" specifier explicitly with any TIMESTAMP column that does not specify ``nullable=False``. In order to accommodate newer databases that specify ``explicit_defaults_for_timestamp``, SQLAlchemy also emits NOT NULL for TIMESTAMP columns that do specify ``nullable=False``. The following example illustrates:: from sqlalchemy import MetaData, Integer, Table, Column, text from sqlalchemy.dialects.mysql import TIMESTAMP m = MetaData() t = Table('ts_test', m, Column('a', Integer), Column('b', Integer, nullable=False), Column('c', TIMESTAMP), Column('d', TIMESTAMP, nullable=False) ) from sqlalchemy import create_engine e = create_engine("mysql+mysqldb://scott:tiger@localhost/test", echo=True) m.create_all(e) output:: CREATE TABLE ts_test ( a INTEGER, b INTEGER NOT NULL, c TIMESTAMP NULL, d TIMESTAMP NOT NULL ) .. versionchanged:: 1.0.0 - SQLAlchemy now renders NULL or NOT NULL in all cases for TIMESTAMP columns, to accommodate ``explicit_defaults_for_timestamp``. Prior to this version, it will not render "NOT NULL" for a TIMESTAMP column that is ``nullable=False``.