如何在T-SQL中删除默认值或类似约束？

Question

我知道语法:

ALTER TABLE [TheTable] DROP CONSTRAINT [TheDefaultConstraint]

但是当我不知道它的名字时,如何删除默认约束？(也就是说,它是在CREATE TABLE时间自动生成的.)

Answer 1

您可以使用此代码自动执行此操作:

DECLARE @tableName VARCHAR(MAX) = '<MYTABLENAME>'
DECLARE @columnName VARCHAR(MAX) = '<MYCOLUMNAME>'
DECLARE @ConstraintName nvarchar(200)
SELECT @ConstraintName = Name 
FROM SYS.DEFAULT_CONSTRAINTS
WHERE PARENT_OBJECT_ID = OBJECT_ID(@tableName) 
AND PARENT_COLUMN_ID = (
    SELECT column_id FROM sys.columns
    WHERE NAME = @columnName AND object_id = OBJECT_ID(@tableName))
IF @ConstraintName IS NOT NULL
    EXEC('ALTER TABLE '+@tableName+' DROP CONSTRAINT ' + @ConstraintName)

只需更换<MYTABLENAME>并<MYCOLUMNNAME>酌情.

Answer 2

如果要手动执行此操作,可以使用Management Studio查找它(在表中的"约束"节点下).

要使用SQL来做到这一点:

• 如果约束是默认约束,您可以使用sys.default_constraints它来查找它:

  SELECT OBJECT_NAME(parent_object_id) AS TableName, name AS ConstraintName
  FROM sys.default_constraints ORDER BY TableName, ConstraintName
  

• 如果您正在寻找其他约束(检查,唯一,外键,默认,主键),您可以使用sysconstraints:

  SELECT OBJECT_NAME(id) AS TableName, OBJECT_NAME(constid) AS ConstraintName
  FROM sysconstraints ORDER BY TableName, ConstraintName
  

您没有说明您使用的是哪个版本的SQL Server.以上是关于SQL 2005和SQL 2008的工作.