How to check if a Constraint exists in Sql server?

I have this sql:

ALTER TABLE dbo.ChannelPlayerSkins
	DROP CONSTRAINT FK_ChannelPlayerSkins_Channels

but apparently, on some other databases we use, the constraint has a different name. How do I check if there's a constraint with the name FK_ChannelPlayerSkins_Channels.

try this:

SELECT
    * 
    FROM INFORMATION_SCHEMA.REFERENTIAL_CONSTRAINTS 
    WHERE CONSTRAINT_NAME ='FK_ChannelPlayerSkins_Channels'

-- EDIT --

When I originally answered this question, I was thinking "Foreign Key" because the original question asked about finding "FK_ChannelPlayerSkins_Channels". Since then many people have commented on finding other "constraints" here are some other queries for that:

--Returns one row for each CHECK, UNIQUE, PRIMARY KEY, and/or FOREIGN KEY
SELECT * 
    FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS
    WHERE CONSTRAINT_NAME='XYZ'  


--Returns one row for each FOREIGN KEY constrain
SELECT * 
    FROM INFORMATION_SCHEMA.REFERENTIAL_CONSTRAINTS 
    WHERE CONSTRAINT_NAME='XYZ'


--Returns one row for each CHECK constraint 
SELECT * 
    FROM INFORMATION_SCHEMA.CHECK_CONSTRAINTS
    WHERE CONSTRAINT_NAME='XYZ'

here is an alternate method

--Returns 1 row for each CHECK, UNIQUE, PRIMARY KEY, FOREIGN KEY, and/or DEFAULT
SELECT 
    OBJECT_NAME(OBJECT_ID) AS NameofConstraint
        ,SCHEMA_NAME(schema_id) AS SchemaName
        ,OBJECT_NAME(parent_object_id) AS TableName
        ,type_desc AS ConstraintType
    FROM sys.objects
    WHERE type_desc LIKE '%CONSTRAINT'
        AND OBJECT_NAME(OBJECT_ID)='XYZ'

If you need even more constraint information, look inside the system stored procedure master.sys.sp_helpconstraint to see how to get certain information. To view the source code using SQL Server Management Studio get into the "Object Explorer". From there you expand the "Master" database, then expand "Programmability", then "Stored Procedures", then "System Stored Procedures". You can then find "sys.sp_helpconstraint" and right click it and select "modify". Just be careful to not save any changes to it. Also, you can just use this system stored procedure on any table by using it like EXEC sp_helpconstraint YourTableNameHere.

Easiest way to check for the existence of a constraint (and then do something such as drop it if it exists) is to use the OBJECT_ID() function...

IF OBJECT_ID('dbo.[CK_ConstraintName]', 'C') IS NOT NULL 
    ALTER TABLE dbo.[tablename] DROP CONSTRAINT CK_ConstraintName

OBJECT_ID can be used without the second parameter ('C' for check constraints only) and that may also work, but if your constraint name matches the name of other objects in the database you may get unexpected results.

IF OBJECT_ID('dbo.[CK_ConstraintName]') IS NOT NULL 
    ALTER TABLE dbo.[tablename] DROP CONSTRAINT CK_ConstraintName

OBJECT_ID can also be used with other "constraints" such as Foreign Key constraints or Primary Key constraints, etc. For best results, always include the appropriate object type as the second parameter for the OBJECT_ID function:

Constraint Object Types:

• C = CHECK constraint
• D = DEFAULT (constraint or stand-alone)
• F = FOREIGN KEY constraint
• PK = PRIMARY KEY constraint
• R = Rule (old-style, stand-alone)
• UQ = UNIQUE constraint

Also note that the schema is often required. The schema of constraints generally takes the schema of the parent table.

Failure to put your constraints (or whatever you are checking) in brackets when using this method may also cause a false negative -- if your object uses unusual characters (such as a .), the brackets are required.

If you are looking for other type of constraint, e.g. defaults, you should use different query (From https://stackoverflow.com/questions/141682/how-do-i-find-a-default-constraint-using-information-schema/142625#142625 answered by devio). Use:

SELECT * FROM sys.objects WHERE type = 'D' AND name = @name

to find a default constraint by name.

I've put together different 'IF not Exists" checks in my post "DDL 'IF not Exists" conditions to make SQL scripts re-runnable"

IF (OBJECT_ID('FK_ChannelPlayerSkins_Channels') IS NOT NULL)

Are you looking at something like this, below is tested in SQL Server 2005

SELECT * FROM sys.check_constraints WHERE 
object_id = OBJECT_ID(N'[dbo].[CK_accounts]') AND 
parent_object_id = OBJECT_ID(N'[dbo]. [accounts]')

Just something to watch out for......

In SQL Server 2008 R2 SSMS, the "Script Constraint as -> DROP And CREATE To" command produces T-SQL like below

USE [MyDatabase]
GO

IF  EXISTS (SELECT * FROM dbo.sysobjects WHERE id = OBJECT_ID(N'[DEF_Detail_IsDeleted]') AND type = 'D')
BEGIN
ALTER TABLE [Patient].[Detail] DROP CONSTRAINT [DEF_Detail_IsDeleted]
END

GO

USE [MyDatabase]
GO

ALTER TABLE [Patient].[Detail] ADD  CONSTRAINT [DEF_Detail_IsDeleted]  DEFAULT ((0)) FOR [IsDeleted]
GO

Out of the box, this script does NOT drop the constraint because the SELECT returns 0 rows. (see post Microsoft Connect).

The name of the default constraint is wrong but I gather it also has something to do with the OBJECT_ID function because changing the name doesn't fix the problem.

To fix this, I removed the usage of OBJECT_ID and used the default constraint name instead.

(SELECT * FROM dbo.sysobjects WHERE [name] = (N'DEF_Detail_IsDeleted') AND type = 'D')

I use the following query to check for an existing constraint before I create it.

IF (NOT EXISTS(SELECT 1 FROM sysconstraints WHERE OBJECT_NAME(constid) = 'UX_CONSTRAINT_NAME' AND OBJECT_NAME(id) = 'TABLE_NAME')) BEGIN
...
END

This queries for the constraint by name targeting a given table name. Hope this helps.

IF EXISTS(SELECT 1 FROM sys.foreign_keys WHERE parent_object_id = OBJECT_ID(N'dbo.TableName'))
 BEGIN 
ALTER TABLE TableName DROP CONSTRAINT CONSTRAINTNAME 
END 

IF EXISTS(SELECT TOP 1 1 FROM sys.default_constraints WHERE parent_object_id = OBJECT_ID(N'[dbo].[ChannelPlayerSkins]') AND name = 'FK_ChannelPlayerSkins_Channels')
BEGIN
    DROP CONSTRAINT FK_ChannelPlayerSkins_Channels
END
GO

INFORMATION_SCHEMA is your friend. It has all kinds of views that show all kinds of schema information. Check your system views. You will find you have three views dealing with constraints, one being CHECK_CONSTRAINTS.

I use this to check for and remote constraints on a column. It should have everything you need.

DECLARE
  @ps_TableName VARCHAR(300)
  , @ps_ColumnName VARCHAR(300)

SET @ps_TableName = 'mytable'
SET @ps_ColumnName = 'mycolumn'

DECLARE c_ConsList CURSOR LOCAL STATIC FORWARD_ONLY FOR
	SELECT
	'ALTER TABLE ' + RTRIM(tb.name) + ' drop constraint ' + sco.name AS csql
	FROM
		sys.Objects tb
		INNER JOIN sys.Columns tc on (tb.Object_id = tc.object_id)
		INNER JOIN sys.sysconstraints sc ON (tc.Object_ID = sc.id and tc.column_id = sc.colid)
		INNER JOIN sys.objects sco ON (sc.Constid = sco.object_id)
	where
		tb.name=@ps_TableName
		AND tc.name=@ps_ColumnName
OPEN c_ConsList
FETCH c_ConsList INTO @ls_SQL
WHILE (@@FETCH_STATUS = 0) BEGIN

	IF RTRIM(ISNULL(@ls_SQL, '')) <> '' BEGIN
		EXECUTE(@ls_SQL)
	END
	FETCH c_ConsList INTO @ls_SQL
END
CLOSE c_ConsList
DEALLOCATE c_ConsList

You can use the one above with one caveat:

IF EXISTS(
    SELECT 1 FROM sys.foreign_keys 
    WHERE parent_object_id = OBJECT_ID(N'dbo.TableName') 
        AND name = 'CONSTRAINTNAME'
)
BEGIN 
    ALTER TABLE TableName DROP CONSTRAINT CONSTRAINTNAME 
END 

Need to use the name = [Constraint name] since a table may have multiple foreign keys and still not have the foreign key being checked for

SELECT tabla.name as Tabla,

		restriccion.name as Restriccion, 
		restriccion.type as Tipo, 
		restriccion.type_desc as Tipo_Desc
FROM {DATABASE_NAME}.sys.objects tabla 

INNER JOIN {DATABASE_NAME}.sys.objects restriccion

ON tabla.object_id = restriccion.parent_object_id

WHERE tabla.type = 'U' - Solo tablas creadas por el usuario.

AND restriccion.type = 'UQ' --Tipo de Restriccion UNIQUE

ORDER BY tabla.name, restriccion.type_desc                

In mySql you need to make sure you are querying the right database! Hence table_schema=DATABASE()

Here are my functions using knex to check if a specific Foreign Key or Index defined in specific database and table

const isFKExists = async (knex, tableName, fkName) => {
  const result = await knex.raw(
    `SELECT COUNT(*) AS 'isExists' FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS WHERE table_schema=DATABASE() AND table_name='${tableName}' AND CONSTRAINT_NAME='${fkName}' AND CONSTRAINT_TYPE = 'FOREIGN KEY'`
  )
  return (result[0][0].isExists === 1)
}

const isIndexExists = async (knex, tableName, indexName) => {
  const result = await knex.raw(
    `SELECT COUNT(*) AS 'isExists' FROM INFORMATION_SCHEMA.STATISTICS WHERE table_schema=DATABASE() AND table_name='${tableName}' AND index_name='${indexName}'`
  )
  return (result[0][0].isExists > 0)
}

Enjoy 