Описание тега coalesce

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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
  ...

References