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|**Azure Database for MySQL**| Fully managed <sup>2</sup> | A relational database service based on the MySQL open-source database engine. A fully managed database-as-a-service (DBaaS) that can handle mission-critical workloads with predictable performance, security, high availability, and dynamic scalability. |
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|**Azure Database for PostgreSQL**| Fully managed <sup>2</sup> | A relational database service based on the PostgreSQL open-source database engine. A fully managed database-as-a-service (DBaaS) that can handle mission-critical workloads with predictable performance, security, high availability, and dynamic scalability. |
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|**Azure Synapse Analytics**| Fully managed <sup>2</sup> |[Azure Synapse Analytics](/azure/synapse-analytics/overview-what-is) is an enterprise analytics service that accelerates time to insight across data warehouses and big data systems. Azure Synapse Analytics brings together the best of SQL technologies used in enterprise data warehousing, Spark technologies used for big data, Data Explorer for log and time series analytics, Pipelines for data integration and ETL/ELT, and deep integration with other Azure services such as Power BI, Cosmos DB, and Azure Machine Learning. The [Azure Synapse SQL](/azure/synapse-analytics/sql/overview-architecture) features provide scale-out architecture for data processing in the form of dedicated SQL pools (formerly SQL DW) and serverless SQL pools. <br /><br />The behavior and Azure portal experience can differ between a dedicated SQL pool in Azure Synapse workspaces, or a standalone dedicated SQL pool (formerly SQL DW) in a logical SQL server. |
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| **[!INCLUDE [fabric](../includes/fabric.md)]** | Fully managed <sup>4</sup> | [Microsoft Fabric](/fabric/get-started/microsoft-fabric-overview) is an all-in-one, Software-as-a-Service analytics solution for enterprises that covers everything from data movement to data science, real-time analytics, data warehousing, and business intelligence. [[!INCLUDE [fabric](../includes/fabric.md)]](/fabric/data-warehouse/data-warehousing) provides multiple database and data warehousing experiences. <br /> - Each [!INCLUDE [fabric](../includes/fabric.md)] Lakehouse automatically includes a **[!INCLUDE [fabric-se](../includes/fabric-se.md)]** to enable data engineers to access a relational layer on top of physical data in the Lakehouse, thanks to automatic schema discovery. <br /> - A Synapse Data Warehouse or **[!INCLUDE [fabric-dw](../includes/fabric-dw.md)]** in [!INCLUDE [fabric](../includes/fabric.md)] provides a "traditional" data warehouse and supports the full transactional T-SQL capabilities you would expect from an enterprise data warehouse. Either data warehousing experience exposes data to analysis and reporting tools using T-SQL/TDS end-point.<br /> - You can mirror an existing [Azure SQL Database into Fabric](/fabric/database/mirrored-database/azure-sql-database).<br /> - [SQL database in [!INCLUDE [fabric](../includes/fabric.md)]](/fabric/database/sql/overview) is a developer-friendly, AI-ready, operational database based on the same SQL Database Engine as found in [Azure SQL Database](/azure/azure-sql/database/sql-database-paas-overview?view=azuresqldb-current&preserve-view=true). For more information, see [Features comparison: Azure SQL Database and Fabric SQL database](/fabric/database/sql/feature-comparison-sql-database-fabric).|
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| **[!INCLUDE [fabric](../includes/fabric.md)]** | Fully managed <sup>4</sup> | [Microsoft Fabric](/fabric/get-started/microsoft-fabric-overview) is an all-in-one, Software-as-a-Service analytics solution for enterprises that covers everything from data movement to data science, real-time analytics, data warehousing, and business intelligence. [[!INCLUDE [fabric](../includes/fabric.md)]](/fabric/data-warehouse/data-warehousing) provides multiple database and data warehousing experiences. <br /> - Each [!INCLUDE [fabric](../includes/fabric.md)] Lakehouse automatically includes a **[!INCLUDE [fabric-se](../includes/fabric-se.md)]** to enable data engineers to access a relational layer on top of physical data in the Lakehouse, thanks to automatic schema discovery. <br /> - A Synapse Data Warehouse or **[!INCLUDE [fabric-dw](../includes/fabric-dw.md)]** in [!INCLUDE [fabric](../includes/fabric.md)] provides a "traditional" data warehouse and supports the full transactional T-SQL capabilities you would expect from an enterprise data warehouse. Either data warehousing experience exposes data to analysis and reporting tools using T-SQL/TDS end-point.<br /> - You can mirror an existing [Azure SQL Database into Fabric](/fabric/database/mirrored-database/azure-sql-database), or create a SQL database in [!INCLUDE [fabric](../includes/fabric.md)]. |
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|**SQL database in [!INCLUDE [fabric](../includes/fabric.md)]**| Fully managed <sup>4</sup> |[SQL database in [!INCLUDE [fabric](../includes/fabric.md)]](/fabric/database/sql/overview) is a developer-friendly, AI-ready, operational database based on the same SQL Database Engine as found in Azure SQL Database. For more information, see [Features comparison: Azure SQL Database and Fabric SQL database](/fabric/database/sql/feature-comparison-sql-database-fabric).|
Specifies the properties of a PRIMARY KEY, FOREIGN KEY, UNIQUE, or CHECK constraint that is part of a new column definition added to a table by using [ALTER TABLE](../../t-sql/statements/alter-table-transact-sql.md).
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Is the name of the constraint. Constraint names must follow the rules for [identifiers](../../relational-databases/databases/database-identifiers.md), except that the name cannot start with a number sign (#). If *constraint_name* is not supplied, a system-generated name is assigned to the constraint.
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#### NULL | NOT NULL
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Specifies whether the column can accept null values. Columns that do not allow null values can be added only if they have a default specified. If the new column allows null values and no default is specified, the new column contains NULL for each row in the table. If the new column allows null values and a default definition is added with the new column, the WITH VALUES option can be used to store the default value in the new column for each existing row in the table.
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Specifies whether the column can accept `NULL` values. Columns that do not allow `NULL` values can be added only if they have a default specified. If the new column allows `NULL` values and no default is specified, the new column contains `NULL` for each row in the table. If the new column allows `NULL` values and a default definition is added with the new column, the WITH VALUES option can be used to store the default value in the new column for each existing row in the table.
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If the new column does not allow null values, a DEFAULT definition must be added with the new column. The new column automatically loads with the default value in the new columns in each existing row.
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If the new column does not allow `NULL` values, a DEFAULT definition must be added with the new column. The new column automatically loads with the default value in the new columns in each existing row.
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When the addition of a column requires physical changes to the data rows of a table, such as adding DEFAULT values to each row, locks are held on the table while ALTER TABLE runs. This affects the ability to change the content of the table while the lock is in place. In contrast, adding a column that allows null values and does not specify a default value is a metadata operation only, and involves no locks.
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When the addition of a column requires physical changes to the data rows of a table, such as adding DEFAULT values to each row, locks are held on the table while ALTER TABLE runs. This affects the ability to change the content of the table while the lock is in place. In contrast, adding a column that allows `NULL` values and does not specify a default value is a metadata operation only, and involves no locks.
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When you use CREATE TABLE or ALTER TABLE, database and session settings influence and possibly override the nullability of the data type that is used in a column definition. We recommend that you always explicitly define noncomputed columns as NULL or NOT NULL or, if you use a user-defined data type, that you allow the column to use the default nullability of the data type. For more information, see [CREATE TABLE](create-table-transact-sql.md).
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When you use `CREATE TABLE` or `ALTER TABLE`, database and session settings influence and possibly override the nullability of the data type that is used in a column definition. We recommend that you always explicitly define noncomputed columns as `NULL` or `NOT NULL` or, if you use a user-defined data type, that you allow the column to use the default nullability of the data type. For more information, see [CREATE TABLE](create-table-transact-sql.md).
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#### PRIMARY KEY
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Is a constraint that enforces entity integrity for a specified column or columns by using a unique index. Only one PRIMARY KEY constraint can be created for each table.
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Corresponding rows are deleted from the referencing table if that row is deleted from the parent table.
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SET NULL
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All the values that make up the foreign key are set to NULL when the corresponding row in the parent table is deleted. For this constraint to execute, the foreign key columns must be nullable.
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All the values that make up the foreign key are set to `NULL` when the corresponding row in the parent table is deleted. For this constraint to execute, the foreign key columns must be nullable.
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SET DEFAULT
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All the values that comprise the foreign key are set to their default values when the corresponding row in the parent table is deleted. For this constraint to execute, all foreign key columns must have default definitions. If a column is nullable and there is no explicit default value set, NULL becomes the implicit default value of the column.
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All the values that comprise the foreign key are set to their default values when the corresponding row in the parent table is deleted. For this constraint to execute, all foreign key columns must have default definitions. If a column is nullable and there is no explicit default value set, `NULL` becomes the implicit default value of the column.
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Do not specify CASCADE if the table will be included in a merge publication that uses logical records. For more information about logical records, see [Group Changes to Related Rows with Logical Records](../../relational-databases/replication/merge/group-changes-to-related-rows-with-logical-records.md).
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The ON DELETE CASCADE cannot be defined if an INSTEAD OF trigger ON DELETE already exists on the table that is being altered.
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`ON DELETE CASCADE` cannot be defined if an `INSTEAD OF` trigger `ON DELETE` already exists on the table that is being altered.
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For example, in the [!INCLUDE[ssSampleDBobject](../../includes/sssampledbobject-md.md)] database, the `ProductVendor` table has a referential relationship with the `Vendor` table. The `ProductVendor`.`VendorID` foreign key references the `Vendor`.`VendorID` primary key.
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## Remarks
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When FOREIGN KEY or CHECK constraints are added, all existing data is verified for constraint violations unless the `WITH NOCHECK` option is specified. If any violations occur, ALTER TABLE fails and an error is returned. When a new PRIMARY KEY or UNIQUE constraint is added to an existing column, the data in the column or columns must be unique. If duplicate values are found, ALTER TABLE fails. The `WITH NOCHECK` option has no effect when PRIMARY KEY or UNIQUE constraints are added.
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Each PRIMARY KEY and UNIQUE constraint generates an index. The number of UNIQUE and PRIMARY KEY constraints cannot cause the number of indexes on the table to exceed 999 nonclustered indexes and 1 clustered index. Foreign key constraints do not automatically generate an index. However, foreign key columns are frequently used in join criteria in queries by matching the column or columns in the foreign key constraint of one table with the primary or unique key column or columns in the other table. An index on the foreign key columns enables the [!INCLUDE[ssDE](../../includes/ssde-md.md)] to quickly find related data in the foreign key table.
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Each PRIMARY KEY and UNIQUE constraint generates an index. The number of UNIQUE and PRIMARY KEY constraints cannot cause the number of indexes on the table to exceed 999 nonclustered indexes and 1 clustered index. Foreign key constraints do not automatically generate an index. However, foreign key columns are frequently used in query join criteria, by matching the foreign key constraint of one table with the primary or unique key column or columns in the other table. An index on the foreign key columns enables the [!INCLUDE[ssDE](../../includes/ssde-md.md)] to quickly find related data in the foreign key table.
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