How to check if a Constraint exists in Sql server?

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 How do I find a default constraint using INFORMATION_SCHEMA? 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)