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CASE

Goes through conditions in sequence and returns the value of the result associated with the matching condition.

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Syntax 1

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

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Parameters

ParameterDescription
conditionAn expression that evaluates to a BOOLEAN value.
resultA value or an expression that evaluates to a value. The CASE expression returns result if its associated condition evaluates to true.

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Returns

  • The result associated with the first condition that evaluates to true. Or,
  • The result in the ELSE clause if no condition evaluates to true. Or,
  • NULL if there is no ELSE clause and no condition evaluates to true.

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Syntax 2

CASE expression
    WHEN value THEN result
    [ ... ]
    [ ELSE result ]
END

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Parameters

ParameterDescription
expressionAn expression that evaluates to a value. expression is computed in the first place and its value will be compared with value in the WHEN clause.
valueA value or an expression that evaluates to a value. Each value is a potential match for the expression.
resultA value or an expression that evaluates to a value. The CASE expression returns result if its associated value matches the expression.

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Returns

  • The result associated with the first value that matches the expression. Or,
  • The result in the ELSE clause if no value matches the expression. Or,
  • NULL if there is no ELSE clause and no value matches the expression.

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Example

The following statement (using Syntax 1) classifies the distance of each trip in the table ‘taxi_trips’ into four levels. 
SELECT id, distance,
  CASE
    WHEN (distance < 3) THEN 'short'
    WHEN (distance >= 3 AND distance < 10) THEN 'mid'
    WHEN (distance >= 10 AND distance < 20) THEN 'long'
    WHEN (distance >= 20) THEN 'extra'
  END AS "Category"
  FROM taxi_trips;

 id | distance | Category
----+----------+----------
  1 |       16 | long
  2 |       23 | extra
  3 |        6 | mid
  4 |        9 | mid
The following statement (using Syntax 2) classifies the distance of each trip according to its digits. 
SELECT id, distance,
  CASE LENGTH (distance::VARCHAR)
    WHEN  1 THEN 'One-digit'
    WHEN  2 THEN 'Double-digit'
    WHEN  3 THEN 'Three-digit'
  END AS Digit
  FROM taxi_trips;

 id | distance |    Digit
----+----------+--------------
  1 |       16 | Double-digit
  2 |       23 | Double-digit
  3 |        6 | One-digit
  4 |        9 | One-digit
(4 rows)

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COALESCE

Returns the first non-null value or null if all values are null. 
COALESCE ( value [ , ... ] )

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NULLIF

Returns null if value1 equals to value2, otherwise returns value1. 
NULLIF ( value1, value2 )

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greatest

This function returns the largest value in a list of expressions. NULL values in the argument list are ignored. The result will be NULL only if all the expressions evaluate to NULL. 
greatest(value1, value2, ...) → ANY

greatest(1, 2, 3) → 3

create table t(id INT, v1 INT2, v2 INT4, v3 INT8);

insert into t values (1, 1, 2, 3), (2, 2, NULL, 5), (3, NULL, NULL, 8), (4, NULL, NULL, NULL);

select greatest(v1, v2, v3) from t order by id;
------ results
3, 5, 8, NULL

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least

This function returns the smallest value in a list of expressions. NULL values in the argument list are ignored. The result will be NULL only if all the expressions evaluate to NULL. 
least(value1, value2, ...) → ANY

create table t(id INT, v1 INT2, v2 INT4, v3 INT8);

insert into t values (1, 1, 2, 3), (2, 2, NULL, 5), (3, NULL, NULL, 8), (4, NULL, NULL, NULL);

select least(1, 2, 3); -- results: 1

select least(v1, v2, v3) from t order by id;
------ results
1, 2, 8, NULL