tsql - How do you drop a default value or similar constraint in T-SQL?

Question

I know the syntax:

ALTER TABLE [TheTable] DROP CONSTRAINT [TheDefaultConstraint]

but how to I drop the default constraint when I don't know its name? (That is, it was autogenerated at CREATE TABLE time.)

Answer 1

You can use this code to do it automatically:

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)

Just replace <MYTABLENAME> and <MYCOLUMNNAME> as appropriate.