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Conditional functions

Amazon S3 Select supports the following conditional functions.

CASE

The CASE expression is a conditional expression, similar to if/then/else statements found in other languages. CASE is used to specify a result when there are multiple conditions. There are two types of CASE expressions: simple and searched.

In simple CASE expressions, an expression is compared with a value. When a match is found, the specified action in the THEN clause is applied. If no match is found, the action in the ELSE clause is applied.

In searched CASE expressions, each CASE is evaluated based on a Boolean expression, and the CASE statement returns the first matching CASE. If no matching CASE is found among the WHEN clauses, the action in the ELSE clause is returned.

Syntax

The following is a simple CASE statement that's used to match conditions:

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

The following is a searched CASE statement that's used to evaluate each condition:

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

Examples

Example 1: Use a simple CASE expression to replace New York City with Big Apple in a query. Replace all other city names with other.

SELECT venuecity, CASE venuecity WHEN 'New York City' THEN 'Big Apple' ELSE 'other' END FROM S3Object;

Query result:

venuecity        |   case
-----------------+-----------
Los Angeles      | other
New York City    | Big Apple
San Francisco    | other
Baltimore        | other
...

Example 2: Use a searched CASE expression to assign group numbers based on the pricepaid value for individual ticket sales:

SELECT pricepaid, CASE WHEN CAST(pricepaid as FLOAT) < 10000 THEN 'group 1' WHEN CAST(pricepaid as FLOAT) > 10000 THEN 'group 2' ELSE 'group 3' END FROM S3Object;					

Query result:

pricepaid |  case
-----------+---------
12624.00 | group 2
10000.00 | group 3
10000.00 | group 3
9996.00 | group 1
9988.00 | group 1
...

COALESCE

COALESCE evaluates the arguments in order and returns the first non-unknown value, that is, the first non-null or non-missing value. This function does not propagate null and missing values.

Syntax

COALESCE ( expression, expression, ... )

Parameters

Examples

COALESCE(1)                -- 1
COALESCE(null)             -- null
COALESCE(null, null)       -- null
COALESCE(missing)          -- null
COALESCE(missing, missing) -- null
COALESCE(1, null)          -- 1
COALESCE(null, null, 1)    -- 1
COALESCE(null, 'string')   -- 'string'
COALESCE(missing, 1)       -- 1

NULLIF

Given two expressions, NULLIF returns NULL if the two expressions evaluate to the same value; otherwise, NULLIF returns the result of evaluating the first expression.

Syntax

NULLIF ( expression1, expression2 )

Parameters

Examples

NULLIF(1, 1)             -- null
NULLIF(1, 2)             -- 1
NULLIF(1.0, 1)           -- null
NULLIF(1, '1')           -- 1
NULLIF([1], [1])         -- null
NULLIF(1, NULL)          -- 1
NULLIF(NULL, 1)          -- null
NULLIF(null, null)       -- null
NULLIF(missing, null)    -- null
NULLIF(missing, missing) -- null