Conditional Functions And Expressions

Synopsis:

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

Input:

One or multiple WHEN condition THEN result clauses are used and the expression can optionally have an ELSE default_result clause. Every condition should be a boolean expression.

Output: one of the result expressions if the corresponding WHEN condition evaluates to true or the default_result if all WHEN condition clauses evaluate to false. If the optional ELSE default_result clause is missing and all WHEN condition clauses evaluate to false then null is returned.

Description: The CASE expression is a generic conditional expression which simulates if/else statements of other programming languages If the condition’s result is true, the value of the result expression that follows the condition will be the returned the subsequent when clauses will be skipped and not processed.

SELECT CASE WHEN 1 > 2 THEN 'elastic'
            WHEN 2 <= 3 THEN 'search'
       END AS "case";

    case
---------------
search

SELECT CASE WHEN 1 > 2 THEN 'elastic'
            WHEN 2 > 10 THEN 'search'
       END AS "case";

    case
---------------
null

SELECT CASE WHEN 1 > 2 THEN 'elastic'
            WHEN 2 > 10 THEN 'search'
            ELSE 'default'
       END AS "case";

    case
---------------
default

As a variant, a case expression can be expressed with a syntax similar to switch-case of other programming languages:

CASE expression
     WHEN value1 THEN result1
    [WHEN value2 THEN result2]
    [WHEN ...]
    [ELSE default_result]
END

In this case it’s transformed internally to:

CASE WHEN expression = value1 THEN result1
    [WHEN expression = value2 THEN result2]
    [WHEN ...]
    [ELSE default_result]
END

SELECT CASE 5
            WHEN 1 THEN 'elastic'
            WHEN 2 THEN 'search'
            WHEN 5 THEN 'elasticsearch'
       END AS "case";

    case
---------------
elasticsearch

SELECT CASE 5
            WHEN 1 THEN 'elastic'
            WHEN 2 THEN 'search'
            WHEN 3 THEN 'elasticsearch'
            ELSE 'default'
       END AS "case";

    case
---------------
default

CASE WHEN a = 1 THEN null
     WHEN a > 2 THEN 10
     WHEN a > 5 THEN 'foo'
END

SELECT count(*) AS count,
  CASE WHEN NVL(languages, 0) = 0 THEN 'zero'
    WHEN languages = 1 THEN 'one'
    WHEN languages = 2 THEN 'bilingual'
    WHEN languages = 3 THEN 'trilingual'
    ELSE 'multilingual'
  END as lang_skills
FROM employees
GROUP BY lang_skills
ORDER BY lang_skills;

Synopsis:

COALESCE(
    expression, 
    expression, 
    ...)

Input:

…​

Nth expression

COALESCE can take an arbitrary number of arguments.

Output: one of the expressions or null

Description: Returns the first of its arguments that is not null. If all arguments are null, then it returns null.

SELECT COALESCE(null, 'elastic', 'search') AS "coalesce";

    coalesce
---------------
elastic

SELECT COALESCE(null, null, null, null) AS "coalesce";

    coalesce
---------------
null

Synopsis:

GREATEST(
    expression, 
    expression, 
    ...)

Input:

…​

Nth expression

GREATEST can take an arbitrary number of arguments and all of them must be of the same data type.

Output: one of the expressions or null

Description: Returns the argument that has the largest value which is not null. If all arguments are null, then it returns null.

SELECT GREATEST(null, 1, 2) AS "greatest";

    greatest
---------------
2

SELECT GREATEST(null, null, null, null) AS "greatest";

    greatest
---------------
null

Synopsis:

IFNULL(
    expression, 
    expression) 

Input:

Output: 2nd expression if 1st expression is null, otherwise 1st expression.

Description: Variant of COALESCE with only two arguments. Returns the first of its arguments that is not null. If all arguments are null, then it returns null.

SELECT IFNULL('elastic', null) AS "ifnull";

    ifnull
---------------
elastic

SELECT IFNULL(null, 'search') AS "ifnull";

    ifnull
---------------
search

Synopsis:

IIF(expression,   
    expression,   
    [expression]) 

Input:

Output: 2nd expression if 1st expression (condition) evaluates to true. If it evaluates to false return 3rd expression. If 3rd expression is not provided return null.

Description: Conditional function that implements the standard IF <condition> THEN <result1> ELSE <result2> logic of programming languages. If the 3rd expression is not provided and the condition evaluates to false, null is returned.

SELECT IIF(1 < 2, 'TRUE', 'FALSE') AS result1, IIF(1 > 2, 'TRUE', 'FALSE') AS result2;

    result1    |    result2
---------------+---------------
TRUE           |FALSE

SELECT IIF(1 < 2, 'TRUE') AS result1, IIF(1 > 2 , 'TRUE') AS result2;

    result1    |    result2
---------------+---------------
TRUE           |null

IIF(a = 1, 'one', IIF(a = 2, 'two', IIF(a = 3, 'three', 'many')))

Synopsis:

ISNULL(
    expression, 
    expression) 

Input:

Output: 2nd expression if 1st expression is null, otherwise 1st expression.

Description: Variant of COALESCE with only two arguments. Returns the first of its arguments that is not null. If all arguments are null, then it returns null.

SELECT ISNULL('elastic', null) AS "isnull";

    isnull
---------------
elastic

SELECT ISNULL(null, 'search') AS "isnull";

    isnull
---------------
search

Synopsis:

LEAST(
    expression, 
    expression, 
    ...)

Input:

…​

Nth expression

LEAST can take an arbitrary number of arguments and all of them must be of the same data type.

Output: one of the expressions or null

Description: Returns the argument that has the smallest value which is not null. If all arguments are null, then it returns null.

SELECT LEAST(null, 2, 1) AS "least";

    least
---------------
1

SELECT LEAST(null, null, null, null) AS "least";

    least
---------------
null

Synopsis:

NULLIF(
    expression, 
    expression) 

Input:

Output: null if the 2 expressions are equal, otherwise the 1st expression.

Description: Returns null when the two input expressions are equal and if not, it returns the 1st expression.

SELECT NULLIF('elastic', 'search') AS "nullif";
    nullif
---------------
elastic

SELECT NULLIF('elastic', 'elastic') AS "nullif";

    nullif:s
---------------
null

Synopsis:

NVL(
    expression, 
    expression) 

Input:

Output: 2nd expression if 1st expression is null, otherwise 1st expression.

Description: Variant of COALESCE with only two arguments. Returns the first of its arguments that is not null. If all arguments are null, then it returns null.

SELECT NVL('elastic', null) AS "nvl";

    nvl
---------------
elastic

SELECT NVL(null, 'search') AS "nvl";

    nvl
---------------
search