检查表的列是否具有唯一约束

Question

I need a query which can tell me if a column of a table has unique constraint or not. If doesn't have, I have to add unique constraint. Currently I am using below query to check if a table's column has unique constraint or not:

IF NOT EXISTS (SELECT *
    FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS TC
    WHERE TC.CONSTRAINT_TYPE = 'UNIQUE' AND CONSTRAINT_NAME = 'IX_Product_Users' AND TABLE_NAME = 'Product_Users')
BEGIN
    ALTER TABLE Product_Users
    ADD CONSTRAINT IX_Product_Users UNIQUE (EmpID)
END
GO

Tried this one too, but it is not able to check on which column the constraint it is:

SELECT * FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS
WHERE TABLE_NAME='Product_Users' AND CONSTRAINT_TYPE = 'UNIQUE'

But I think this is a wrong way cause there might be possibility that unique constraint name is different. Is there any precise way to do this?

Answer 1

Rather than using the constraint name look for the same definition. something like

SELECT * 
FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS tc 
    inner join INFORMATION_SCHEMA.CONSTRAINT_COLUMN_USAGE cu 
        on cu.CONSTRAINT_NAME = tc.CONSTRAINT_NAME 
where 
    tc.CONSTRAINT_TYPE = 'UNIQUE'
    and tc.TABLE_NAME = 'Product_Users'
    and cu.COLUMN_NAME = 'EmpID'

Answer 2

I would like to suggest this query:

SELECT tc.CONSTRAINT_NAME, count(*)
FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS tc 
    inner join INFORMATION_SCHEMA.CONSTRAINT_COLUMN_USAGE cu 
        on cu.CONSTRAINT_NAME = tc.CONSTRAINT_NAME 
where 
    tc.CONSTRAINT_TYPE = 'UNIQUE'
    and tc.TABLE_NAME = 'Product_Users'
    and cu.COLUMN_NAME = 'EmpID'
group by tc.CONSTRAINT_NAME
having count(*) = 1

The column can be envolved in a unique constraint with multiple columns, this way you will obtain the unique constrains that only have your column.