6.3. Conditional Expressions

6.3. Conditional Expressions

CASE

The standard SQL CASE expression has two forms.The “simple” form searches each value expression from left to rightuntil 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 isreturned 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 leftto 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 isreturned 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 constructthat 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(value1, value2[, …])
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.

TRY

try(expression)
Evaluate an expression and handle certain types of errors by returningNULL.

In cases where it is preferable that queries produce NULL or default valuesinstead of failing when corrupt or invalid data is encountered, the TRYfunction may be useful. To specify default values, the TRY function can beused in conjunction with the COALESCE function.

The following errors are handled by TRY:

Division by zero
Invalid cast or function argument
Numeric value out of range

Examples

Source table with some invalid data:

SELECT * FROM shipping;

 origin_state | origin_zip | packages | total_cost
--------------+------------+----------+------------
 California   |      94131 |       25 |        100
 California   |      P332a |        5 |         72
 California   |      94025 |        0 |        155
 New Jersey   |      08544 |      225 |        490
(4 rows)

Query failure without TRY:

SELECT CAST(origin_zip AS BIGINT) FROM shipping;

Query failed: Can not cast 'P332a' to BIGINT

NULL values with TRY:

SELECT TRY(CAST(origin_zip AS BIGINT)) FROM shipping;

 origin_zip

NULL 94025 08544(4 rows)

Query failure without TRY:

SELECT total_cost / packages AS per_package FROM shipping;

Query failed: / by zero

Default values with TRY and COALESCE:

SELECT COALESCE(TRY(total_cost / packages), 0) AS per_package FROM shipping;

 per_package

(4 rows)