If a primary key is defined on a CLR user-defined type column, the implementation of the type must support binary ordering.Ī foreign key (FK) is a column or combination of columns that is used to establish and enforce a link between the data in two tables to control the data that can be stored in the foreign key table. If nullability is not specified, all columns participating in a primary key constraint have their nullability set to not null. If clustered or nonclustered is not specified for a primary key constraint, clustered is used if there is no clustered index on the table.Īll columns defined within a primary key constraint must be defined as not null. The index generated by a primary key constraint cannot cause the number of indexes on the table to exceed 999 nonclustered indexes and 1 clustered index. This prevents the insertion of duplicate rows.Ī table can contain only one primary key constraint.Ī primary key cannot exceed 16 columns and a total key length of 900 bytes. This makes sure that every row in the ProductVendor table has a unique combination of ProductID and VendorID. If a primary key constraint is defined on more than one column, values may be duplicated within one column, but each combination of values from all the columns in the primary key constraint definition must be unique.Īs shown in the following illustration, the ProductID and VendorID columns in the Purchasing.ProductVendor table form a composite primary key constraint for this table. This index also permits fast access to data when the primary key is used in queries. When you specify a primary key constraint for a table, the Database Engine enforces data uniqueness by automatically creating a unique index for the primary key columns. Because primary key constraints guarantee unique data, they are frequently defined on an identity column. This column, or columns, is called the primary key (PK) of the table and enforces the entity integrity of the table. Primary Key ConstraintsĪ table typically has a column or combination of columns that contain values that uniquely identify each row in the table. Primary keys and foreign keys are two types of constraints that can be used to enforce data integrity in SQL Server tables. If you are on latest version of MySQL you can add check constraint to avoid issues when both of these columns are nulls or both of them are not null simultaneously.SQL Server 2016 (13.x) and later Azure SQL Database Azure SQL Managed Instance You will need to insert foreignId value into one of these columns (foreignIdMultipleChoice or foreignIdText) depending on the field source. ,CONSTRAINT FK_foreignIdText FOREIGN KEY (foreignIdText) REFERENCES customFieldsMultipleChoice(id) ,CONSTRAINT FK_foreignIdMultipleChoice FOREIGN KEY (foreignIdMultipleChoice) ,foreignId INT AS (IFNULL(foreignIdMultipleChoice, foreignIdText)) ,type VARCHAR(15) NOT NULL - it would be better to use bit Maybe it's not exactly what you expected but you can try such table structure: CREATE TABLE customFields ( How would you solve this issue? Is there a way to create such a foreign key constraint? I inherited the two tables mentioned above so I'd be disinclined to change their schema. I think it's probably clear what I want to achieve here: I want to be able to uniquely identify all my customFields by a single unsigned integer key. I'm not too experienced with SQL and I don't know if this is possible at all. I would like to have "something like" a foreign key constraint that works this way: if type = 'text' thenĮnforce that foreignId can be found in customFieldsText tableĮnforce that the foreignId can be found in the customFieldsMultipleChoice table I'd like to enforce that is a unique key on this table. I'd like to create a new table called customFields with the following two columns: customFields Both tables have an auto-incremental unsigned int primary key. I have two MySQL tables customFieldsText and customFieldsMultipleChoice.
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