# Conditional expressions#

## 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 expression has two forms, one supplying only a true_value and the other supplying both a true_value and a false_value:

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.

The following IF and CASE expressions are equivalent:

SELECT
orderkey,
totalprice,
IF(totalprice >= 150000, 'High Value', 'Low Value')
FROM tpch.sf1.orders;

SELECT
orderkey,
totalprice,
CASE
WHEN totalprice >= 150000 THEN 'High Value'
ELSE 'Low Value'
END
FROM tpch.sf1.orders;


## 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 returning NULL.

In cases where it is preferable that queries produce NULL or default values instead of failing when corrupt or invalid data is encountered, the TRY function may be useful. To specify default values, the TRY function can be used 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: Cannot cast 'P332a' to BIGINT


NULL values with TRY:

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

 origin_zip
------------
94131
NULL
94025
08544
(4 rows)


Query failure without TRY:

SELECT total_cost / packages AS per_package FROM shipping;

Query failed: Division by zero


Default values with TRY and COALESCE:

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

 per_package
-------------
4
14
0
19
(4 rows)