COALESCE

Syntax#

COALESCE([Column1],[Column2]…[ColumnN]

Using COALESCE to Build Comma-Delimited String

We can get a comma delimited string from multiple rows using coalesce as shown below.

Since table variable is used, we need to execute whole query once. So to make easy to understand, I have added BEGIN and END block.

BEGIN

    --Table variable declaration to store sample records
    DECLARE @Table TABLE (FirstName varchar(256), LastName varchar(256))

    --Inserting sample records into table variable @Table
    INSERT INTO @Table (FirstName, LastName)
    VALUES
    ('John','Smith'),
    ('Jane','Doe')

    --Creating variable to store result          
    DECLARE @Names varchar(4000)

    --Used COLESCE function, so it will concatenate comma seperated FirstName into @Names varible
    SELECT @Names = COALESCE(@Names + ',', '') + FirstName
    FROM @Table

    --Now selecting actual result 
    SELECT @Names
    END

Coalesce basic Example

COALESCE() returns the first NON NULL value in a list of arguments. Suppose we had a table containing phone numbers, and cell phone numbers and wanted to return only one for each user. In order to only obtain one, we can get the first NON NULL value.

DECLARE @Table TABLE (UserID int, PhoneNumber varchar(12), CellNumber varchar(12))
INSERT INTO @Table (UserID, PhoneNumber, CellNumber)
VALUES
(1,'555-869-1123',NULL),
(2,'555-123-7415','555-846-7786'),
(3,NULL,'555-456-8521')


SELECT
    UserID,
    COALESCE(PhoneNumber, CellNumber)
FROM
    @Table

Getting the first not null from a list of column values

SELECT COALESCE(NULL, NULL, 'TechOnTheNet.com', NULL, 'CheckYourMath.com');
Result: 'TechOnTheNet.com'

SELECT COALESCE(NULL, 'TechOnTheNet.com', 'CheckYourMath.com');
Result: 'TechOnTheNet.com'

SELECT COALESCE(NULL, NULL, 1, 2, 3, NULL, 4);
Result: 1