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Case Statement

The CASE statement performs a switch based on a condition. The basic form is identical to the ternary condition used in many programming languages (CASE WHEN cond THEN a ELSE b END is equivalent to cond ? a : b). With a single condition this can be expressed with IF(cond, a, b).

CREATE OR REPLACE TABLE INTEGERS as SELECT UNNEST([1, 2, 3]) as i;
SELECT i, CASE WHEN i>2 THEN 1 ELSE 0 END AS test FROM integers;
-- 1, 2, 3
-- 0, 0, 1

-- this is equivalent to:
SELECT i, IF(i > 2, 1, 0) AS test FROM integers;
-- 1, 2, 3
-- 0, 0, 1

The WHEN cond THEN expr part of the CASE statement can be chained, whenever any of the conditions returns true for a single tuple, the corresponding expression is evaluated and returned.

CREATE OR REPLACE TABLE INTEGERS as SELECT UNNEST([1, 2, 3]) as i;
SELECT i, CASE WHEN i=1 THEN 10 WHEN i=2 THEN 20 ELSE 0 END AS test FROM integers;
-- 1, 2, 3
-- 10, 20, 0

The ELSE part of the CASE statement is optional. If no else statement is provided and none of the conditions match, the CASE statement will return NULL.

CREATE OR REPLACE TABLE INTEGERS as SELECT UNNEST([1, 2, 3]) as i;
SELECT i, CASE WHEN i=1 THEN 10 END AS test FROM integers;
-- 1, 2, 3
-- 10, NULL, NULL

After the CASE but before the WHEN an individual expression can also be provided. When this is done, the CASE statement is essentially transformed into a switch statement.

CREATE OR REPLACE TABLE INTEGERS as SELECT UNNEST([1, 2, 3]) as i;
SELECT i, CASE i WHEN 1 THEN 10 WHEN 2 THEN 20 WHEN 3 THEN 30 END AS test FROM integers;
-- 1, 2, 3
-- 10, 20, 30

-- this is equivalent to:
SELECT i, CASE WHEN i=1 THEN 10 WHEN i=2 THEN 20 WHEN i=3 THEN 30 END AS test FROM integers;
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