外键Foreign Keys

查询外键是否存在

以SQL Server为例，查询外键是否存在，如果不存在，则创建外键

IF NOT EXISTS (SELECT *
FROM sys.foreign_keys fk
INNER JOIN sys.tables fk_tab
ON fk_tab.object_id = fk.parent_object_id
INNER JOIN sys.tables pk_tab
ON pk_tab.object_id = fk.referenced_object_id
WHERE fk.name ='FK_NAME'
AND pk_tab.name ='PK_Table_Name'
AND fk_tab.name ='FK_Table_Name')
ALTER TABLE dbo.FK_Table_Name ADD CONSTRAINT FK_NAME FOREIGN KEY (FK_Column_Name) REFERENCES dbo.PK_Table_Name(PK_Column_Name)
GO

参考资料

List foreign keys in SQL Server database