存储过程：变量表名称

Question

I want to make it easier to insert new default values to existing columns from my setup. For better overview and less errors, I want to make a oneliner from a three-liner with redundant names. My first attempt at a store dprocedure for that is:

CREATE PROCEDURE InsertDefault
    -- Add the parameters for the stored procedure here
    @TableName VARCHAR(200),
    @FieldName VARCHAR(200),
    @DefaultValue VARCHAR(200)
AS
BEGIN
    IF NOT EXISTS (SELECT * FROM sys.columns WHERE object_id = OBJECT_ID(@TableName) AND name = @FieldName)
        ALTER TABLE @TableName ADD @FieldName [int] CONSTRAINT CONCAT('DF_', @TableName, '_', @FieldName) DEFAULT @DefaultValue NOT NULL
    GO
END
GO

But it seems that ALTER TABLE @TableName does not work, nor does the concatenation for the constraint name.

What am I missing here?

And then, is there a possibility to make @DefaultValue a mixed type?

Answer 1

You have to use dynamic sql (don't forget to escape single quotes in query string if they are present):

declare @cmd nvarchar(4000)
select @cmd = 'ALTER TABLE ' + @TableName + ' ADD ' + @FieldName 
       + ' [int] CONSTRAINT ' + CONCAT('DF_', @TableName, '_', @FieldName) 
       + ' DEFAULT ' + @DefaultValue + ' NOT NULL'
exec sp_executesql @cmd

Answer 2

Try this....

BEGIN
    IF NOT EXISTS (SELECT * FROM sys.columns WHERE object_id = OBJECT_ID(@TableName) AND name = @FieldName)
        exec('ALTER TABLE ' + @TableName + 'ADD ' + @FieldName + ' [int] CONSTRAINT CONCAT(''DF_'', ' + @TableName + ','' _'', ' + @FieldName + ') DEFAULT ' + @DefaultValue + ' NOT NULL')
    GO
END