CASE

The standard SQL CASE expression has two forms. The “simple” form searches each value expression from left to right until it finds one that equals expression:

CASE expression
    WHEN value THEN result
    [ WHEN ... ]
    [ ELSE result ]
END

The result for the matching value is returned. If no match is found, the result from the ELSE clause is returned if it exists, otherwise null is returned. Example:

SELECT a,
       CASE a
           WHEN 1 THEN 'one'
           WHEN 2 THEN 'two'
           ELSE 'many'
       END

The “searched” form evaluates each boolean condition from left to right until one is true and returns the matching result:

CASE
    WHEN condition THEN result
    [ WHEN ... ]
    [ ELSE result ]
END

If no conditions are true, the result from the ELSE clause is returned if it exists, otherwise null is returned. Example:

SELECT a, b,
       CASE
           WHEN a = 1 THEN 'aaa'
           WHEN b = 2 THEN 'bbb'
           ELSE 'ccc'
       END

IF

The IF function is actually a language construct that is equivalent to the following CASE expression:

CASE
    WHEN condition THEN true_value
    [ ELSE false_value ]
END

if(condition, true_value)

Evaluates and returns true_value if condition is true, otherwise null is returned and true_value is not evaluated.

if(condition, true_value, false_value)

Evaluates and returns true_value if condition is true, otherwise evaluates and returns false_value.

COALESCE

coalesce(value[, ...])

Returns the first non-null value in the argument list. Like a CASE expression, arguments are only evaluated if necessary.

NULLIF

nullif(value1, value2)

Returns null if value1 equals value2, otherwise returns value1.