[英]If Foreign Key Not Exist Then Add Foreign Key Constraint(Or Drop a Foreign Key Constraint If Exist) without using Name?
我發現創建查詢有困難。 假設我有一個產品和品牌表。 我可以使用此命令添加外鍵,
ALTER TABLE Products
ADD FOREIGN KEY (BrandID)
REFERENCES Brands(ID)
但是如果外鍵不存在,我只需要運行這個命令。 我需要的類似的事情是在不使用名稱的情況下刪除外鍵約束如果存在。
試試這個:
IF NOT EXISTS (SELECT * FROM sys.objects o WHERE o.object_id = object_id(N'[dbo].[FK_Products_Brands]') AND OBJECTPROPERTY(o.object_id, N'IsForeignKey') = 1)
BEGIN
ALTER TABLE [dbo].[Products] WITH CHECK ADD CONSTRAINT [FK_Products_Brands] FOREIGN KEY([BrandID]) REFERENCES [dbo].[Brands] ([Id])
END
首先,你應該總是命名你的FK和所有其他約束,以便像這樣省去自己的麻煩。
但是,如果您不知道FK的名稱,可以使用多個系統視圖進行檢查:
IF NOT EXISTS
(
SELECT * FROM sys.foreign_key_columns fk
INNER JOIN sys.columns pc ON pc.object_id = fk.parent_object_id AND pc.column_id = fk.parent_column_id
INNER JOIN sys.columns rc ON rc.object_id = fk.referenced_object_id AND rc.column_id = fk.referenced_column_id
WHERE fk.parent_object_id = object_id('Products') AND pc.name = 'BrandID'
AND fk.referenced_object_id = object_id('Brands') AND rc.NAME = 'ID'
)
ALTER TABLE Products
ADD CONSTRAINT Your_New_FK_NAME FOREIGN KEY (BrandID)
REFERENCES Brands(ID)
你也可以使用它。
IF(OBJECT_ID('FK_Products_Brands', 'F') IS NULL)
ALTER TABLE [dbo].[Products] WITH CHECK ADD CONSTRAINT [FK_Products_Brands] FOREIGN KEY([BrandID]) REFERENCES [dbo].[Brands] ([Id])
要在不知道約束名稱且沒有內部聯接的情況下執行此操作,您可以執行以下操作:
IF NOT EXISTS(SELECT NULL FROM INFORMATION_SCHEMA.CONSTRAINT_COLUMN_USAGE WHERE [TABLE_NAME] = 'Products' AND [COLUMN_NAME] = 'BrandID')
BEGIN
ALTER TABLE Products
ADD FOREIGN KEY (BrandID)
REFERENCES Brands(ID)
END
如果您願意,可以從此表中獲取約束的名稱,然后執行刪除/添加。
這在 Azure sql 服務器中對我來說很順利。
IF NOT EXISTS (
SELECT NULL
FROM information_schema.TABLE_CONSTRAINTS
WHERE
CONSTRAINT_SCHEMA = 'dbo' AND
CONSTRAINT_NAME = 'FK_company_id' AND
CONSTRAINT_TYPE = 'FOREIGN KEY'
) ALTER TABLE dbo.table_main
ADD CONSTRAINT FK_company_id
FOREIGN KEY (company_id)
REFERENCES dbo.table_company(id);
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