本地英文版地址: ../en/sql-functions-conditional.html
Conditional Functions And Expressionsedit
Functions that return one of their arguments by evaluating in an if-else manner.
CASEedit
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
All result expressions must be of compatible data types. More specifically all result expressions should have a compatible data type with the 1st non-null result expression. E.g.:
for the following query:
CASE WHEN a = 1 THEN null
WHEN a > 2 THEN 10
WHEN a > 5 THEN 'foo'
END
an error message would be returned, mentioning that foo is of data type keyword, which does not match the expected data type integer (based on result 10).
Conditional bucketingedit
CASE can be used as a GROUP BY key in a query to facilitate custom bucketing and assign descriptive names to those buckets. If, for example, the values for a key are too many or, simply, ranges of those values are more interesting than every single value, CASE can create custom buckets as in the following example:
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;
With this query, one can create normal grouping buckets for values 0, 1, 2, 3 with descriptive names, and every value >= 4 falls into the multilingual bucket.
COALESCEedit
Synopsis:
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
GREATESTedit
Synopsis:
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
IFNULLedit
Synopsis:
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
IIFedit
Synopsis:
Input:
|
boolean condition to check |
|
|
return value if the boolean condition evaluates to |
|
|
return value if the boolean condition evaluates |
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 functions can be combined to implement more complex logic simulating the CASE
expression. E.g.:
IIF(a = 1, 'one', IIF(a = 2, 'two', IIF(a = 3, 'three', 'many')))
ISNULLedit
Synopsis:
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
LEASTedit
Synopsis:
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
NULLIFedit
Synopsis:
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
NVLedit
Synopsis:
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