COALESCE is a SQL function that returns the first non-NULL expression among its arguments. COALESCE() is ANSI standard and may be used instead of vendor-specific alternatives such as ISNULL(), NVL() or IFNULL().
Return the first non-NULL argument. The COALESCE(expression1, expression2... expressionN) function is variety of the CASE expression.
Examples
Consider the expression:
CASE
WHEN (expression1 IS NOT NULL) THEN expression1
WHEN (expression2 IS NOT NULL) THEN expression2
...
ELSE expressionN
It is equivalent to
COALESCE(expression1, ... expressionN);
Another usage:
SELECT
field1,
COALESCE(SUM(customers), 0), -- if the SUM is NULL, it would be calculated as 0
field3,
FROM
...
