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Showing content from https://cloud.google.com/spanner/docs/reference/standard-sql/conditional_expressions below:

Conditional expressions in GoogleSQL | Spanner

GoogleSQL for Spanner supports conditional expressions. Conditional expressions impose constraints on the evaluation order of their inputs. In essence, they are evaluated left to right, with short-circuiting, and only evaluate the output value that was chosen. In contrast, all inputs to regular functions are evaluated before calling the function. Short-circuiting in conditional expressions can be exploited for error handling or performance tuning.

CASE expr
  WHEN expr_to_match THEN result
  [ ... ]
  [ ELSE else_result ]
  END

Description

Compares expr to expr_to_match of each successive WHEN clause and returns the first result where this comparison evaluates to TRUE. The remaining WHEN clauses and else_result aren't evaluated.

If the expr = expr_to_match comparison evaluates to FALSE or NULL for all WHEN clauses, returns the evaluation of else_result if present; if else_result isn't present, then returns NULL.

Consistent with equality comparisons elsewhere, if both expr and expr_to_match are NULL, then expr = expr_to_match evaluates to NULL, which returns else_result. If a CASE statement needs to distinguish a NULL value, then the alternate CASE syntax should be used.

expr and expr_to_match can be any type. They must be implicitly coercible to a common supertype; equality comparisons are done on coerced values. There may be multiple result types. result and else_result expressions must be coercible to a common supertype.

Return Data Type

Supertype of result[, ...] and else_result.

Example

WITH Numbers AS (
  SELECT 90 as A, 2 as B UNION ALL
  SELECT 50, 8 UNION ALL
  SELECT 60, 6 UNION ALL
  SELECT 50, 10
)
SELECT
  A,
  B,
  CASE A
    WHEN 90 THEN 'red'
    WHEN 50 THEN 'blue'
    ELSE 'green'
    END
    AS result
FROM Numbers

/*------------------*
 | A  | B  | result |
 +------------------+
 | 90 | 2  | red    |
 | 50 | 8  | blue   |
 | 60 | 6  | green  |
 | 50 | 10 | blue   |
 *------------------*/

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

Description

Evaluates the condition of each successive WHEN clause and returns the first result where the condition evaluates to TRUE; any remaining WHEN clauses and else_result aren't evaluated.

If all conditions evaluate to FALSE or NULL, returns evaluation of else_result if present; if else_result isn't present, then returns NULL.

For additional rules on how values are evaluated, see the three-valued logic table in Logical operators.

condition must be a boolean expression. There may be multiple result types. result and else_result expressions must be implicitly coercible to a common supertype.

Return Data Type

Supertype of result[, ...] and else_result.

Example

WITH Numbers AS (
  SELECT 90 as A, 2 as B UNION ALL
  SELECT 50, 6 UNION ALL
  SELECT 20, 10
)
SELECT
  A,
  B,
  CASE
    WHEN A > 60 THEN 'red'
    WHEN B = 6 THEN 'blue'
    ELSE 'green'
    END
    AS result
FROM Numbers

/*------------------*
 | A  | B  | result |
 +------------------+
 | 90 | 2  | red    |
 | 50 | 6  | blue   |
 | 20 | 10 | green  |
 *------------------*/

COALESCE(expr[, ...])

Description

Returns the value of the first non-NULL expression, if any, otherwise NULL. The remaining expressions aren't evaluated. An input expression can be any type. There may be multiple input expression types. All input expressions must be implicitly coercible to a common supertype.

Return Data Type

Supertype of expr[, ...].

Examples

SELECT COALESCE('A', 'B', 'C') as result

/*--------*
 | result |
 +--------+
 | A      |
 *--------*/

SELECT COALESCE(NULL, 'B', 'C') as result

/*--------*
 | result |
 +--------+
 | B      |
 *--------*/

IF(expr, true_result, else_result)

Description

If expr evaluates to TRUE, returns true_result, else returns the evaluation for else_result. else_result isn't evaluated if expr evaluates to TRUE. true_result isn't evaluated if expr evaluates to FALSE or NULL.

expr must be a boolean expression. true_result and else_result must be coercible to a common supertype.

Return Data Type

Supertype of true_result and else_result.

Examples

SELECT
  10 AS A,
  20 AS B,
  IF(10 < 20, 'true', 'false') AS result

/*------------------*
 | A  | B  | result |
 +------------------+
 | 10 | 20 | true   |
 *------------------*/

SELECT
  30 AS A,
  20 AS B,
  IF(30 < 20, 'true', 'false') AS result

/*------------------*
 | A  | B  | result |
 +------------------+
 | 30 | 20 | false  |
 *------------------*/

IFNULL(expr, null_result)

Description

If expr evaluates to NULL, returns null_result. Otherwise, returns expr. If expr doesn't evaluate to NULL, null_result isn't evaluated.

expr and null_result can be any type and must be implicitly coercible to a common supertype. Synonym for COALESCE(expr, null_result).

Return Data Type

Supertype of expr or null_result.

Examples

SELECT IFNULL(NULL, 0) as result

/*--------*
 | result |
 +--------+
 | 0      |
 *--------*/

SELECT IFNULL(10, 0) as result

/*--------*
 | result |
 +--------+
 | 10     |
 *--------*/

NULLIF(expr, expr_to_match)

Description

Returns NULL if expr = expr_to_match evaluates to TRUE, otherwise returns expr.

expr and expr_to_match must be implicitly coercible to a common supertype, and must be comparable.

Return Data Type

Supertype of expr and expr_to_match.

Example

SELECT NULLIF(0, 0) as result

/*--------*
 | result |
 +--------+
 | NULL   |
 *--------*/

SELECT NULLIF(10, 0) as result

/*--------*
 | result |
 +--------+
 | 10     |
 *--------*/

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