459 lines
9.7 KiB
Plaintext
459 lines
9.7 KiB
Plaintext
[role="xpack"]
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[testenv="basic"]
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[[sql-functions-conditional]]
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=== Conditional Functions And Expressions
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Functions that return one of their arguments by evaluating in an if-else manner.
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[[sql-functions-conditional-case]]
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==== `CASE`
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.Synopsis:
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[source, sql]
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----
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CASE WHEN condition THEN result
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[WHEN ...]
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[ELSE default_result]
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END
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----
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*Input*:
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One or multiple _WHEN *condition* THEN *result_* clauses are used and the expression can optionally have
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an _ELSE *default_result_* clause. Every *condition* should be a boolean expression.
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*Output*: one of the *result* expressions if the corresponding _WHEN *condition_* evaluates to `true` or
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the *default_result* if all _WHEN *condition_* clauses evaluate to `false`. If the optional _ELSE *default_result_*
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clause is missing and all _WHEN *condition_* clauses evaluate to `false` then `null` is returned.
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.Description
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The CASE expression is a generic conditional expression which simulates if/else statements of other programming languages
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If the condition’s result is true, the value of the result expression that follows the condition will be the returned
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the subsequent when clauses will be skipped and not processed.
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["source","sql",subs="attributes,callouts,macros"]
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----
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include-tagged::{sql-specs}/docs/docs.csv-spec[case]
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----
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["source","sql",subs="attributes,callouts,macros"]
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----
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include-tagged::{sql-specs}/docs/docs.csv-spec[caseReturnNull]
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----
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["source","sql",subs="attributes,callouts,macros"]
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----
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include-tagged::{sql-specs}/docs/docs.csv-spec[caseWithElse]
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----
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As a variant, a case expression can be expressed with a syntax similar to *switch-case* of other programming languages:
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[source, sql]
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----
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CASE expression
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WHEN value1 THEN result1
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[WHEN value2 THEN result2]
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[WHEN ...]
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[ELSE default_result]
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END
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----
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In this case it's transformed internally to:
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[source, sql]
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----
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CASE WHEN expression = value1 THEN result1
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[WHEN expression = value2 THEN result2]
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[WHEN ...]
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[ELSE default_result]
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END
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----
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["source","sql",subs="attributes,callouts,macros"]
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----
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include-tagged::{sql-specs}/docs/docs.csv-spec[caseWithOperand]
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----
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["source","sql",subs="attributes,callouts,macros"]
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----
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include-tagged::{sql-specs}/docs/docs.csv-spec[caseWithOperandAndElse]
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----
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[NOTE]
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===============================
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All result expressions must be of compatible data types. More specifically all result
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expressions should have a compatible data type with the 1st _non-null_ result expression.
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E.g.:
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for the following query:
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[source, sql]
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CASE WHEN a = 1 THEN null
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WHEN a > 2 THEN 10
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WHEN a > 5 THEN 'foo'
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END
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an error message would be returned, mentioning that *'foo'* is of data type *keyword*,
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which does not match the expected data type *integer* (based on result *10*).
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===============================
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[[sql-functions-conditional-case-groupby-custom-buckets]]
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===== Conditional bucketing
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CASE can be used as a GROUP BY key in a query to facilitate custom bucketing
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and assign descriptive names to those buckets. If, for example, the values
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for a key are too many or, simply, ranges of those values are more
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interesting than every single value, CASE can create custom buckets as in the
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following example:
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[source, sql]
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SELECT count(*) AS count,
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CASE WHEN NVL(languages, 0) = 0 THEN 'zero'
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WHEN languages = 1 THEN 'one'
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WHEN languages = 2 THEN 'bilingual'
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WHEN languages = 3 THEN 'trilingual'
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ELSE 'multilingual'
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END as lang_skills
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FROM employees
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GROUP BY lang_skills
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ORDER BY lang_skills;
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With this query, one can create normal grouping buckets for values _0, 1, 2, 3_ with
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descriptive names, and every value _>= 4_ falls into the _multilingual_ bucket.
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[[sql-functions-conditional-coalesce]]
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==== `COALESCE`
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.Synopsis:
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[source, sql]
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----
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COALESCE(
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expression, <1>
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expression, <2>
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...)
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----
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*Input*:
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<1> 1st expression
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<2> 2nd expression
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...
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**N**th expression
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COALESCE can take an arbitrary number of arguments.
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*Output*: one of the expressions or `null`
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.Description
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Returns the first of its arguments that is not null.
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If all arguments are null, then it returns `null`.
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["source","sql",subs="attributes,callouts,macros"]
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----
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include-tagged::{sql-specs}/docs/docs.csv-spec[coalesceReturnNonNull]
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----
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["source","sql",subs="attributes,callouts,macros"]
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----
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include-tagged::{sql-specs}/docs/docs.csv-spec[coalesceReturnNull]
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----
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[[sql-functions-conditional-greatest]]
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==== `GREATEST`
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.Synopsis:
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[source, sql]
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----
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GREATEST(
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expression, <1>
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expression, <2>
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...)
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----
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*Input*:
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<1> 1st expression
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<2> 2nd expression
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...
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**N**th expression
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GREATEST can take an arbitrary number of arguments and
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all of them must be of the same data type.
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*Output*: one of the expressions or `null`
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.Description
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Returns the argument that has the largest value which is not null.
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If all arguments are null, then it returns `null`.
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["source","sql",subs="attributes,callouts,macros"]
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----
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include-tagged::{sql-specs}/docs/docs.csv-spec[greatestReturnNonNull]
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----
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["source","sql",subs="attributes,callouts,macros"]
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----
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include-tagged::{sql-specs}/docs/docs.csv-spec[greatestReturnNull]
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----
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[[sql-functions-conditional-ifnull]]
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==== `IFNULL`
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.Synopsis:
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[source, sql]
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----
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IFNULL(
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expression, <1>
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expression) <2>
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----
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*Input*:
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<1> 1st expression
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<2> 2nd expression
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*Output*: 2nd expression if 1st expression is null, otherwise 1st expression.
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.Description
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Variant of <<sql-functions-conditional-coalesce>> with only two arguments.
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Returns the first of its arguments that is not null.
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If all arguments are null, then it returns `null`.
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["source","sql",subs="attributes,callouts,macros"]
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----
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include-tagged::{sql-specs}/docs/docs.csv-spec[ifNullReturnFirst]
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----
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["source","sql",subs="attributes,callouts,macros"]
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----
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include-tagged::{sql-specs}/docs/docs.csv-spec[ifNullReturnSecond]
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----
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[[sql-functions-conditional-iif]]
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==== `IFF`
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.Synopsis:
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[source, sql]
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----
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IIF(expression, <1>
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expression, <2>
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[expression]) <3>
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----
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*Input*:
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<1> boolean condition to check
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<2> return value if the boolean condition evaluates to `true`
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<3> return value if the boolean condition evaluates `false`; optional
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*Output*: 2nd expression if 1st expression (condition) evaluates to `true`. If it evaluates to `false`
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return 3rd expression. If 3rd expression is not provided return `null`.
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.Description
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Conditional function that implements the standard _IF <condition> THEN <result1> ELSE <result2>_
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logic of programming languages. If the 3rd expression is not provided and the condition evaluates to `false`,
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`null` is returned.
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["source","sql",subs="attributes,callouts,macros"]
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----
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include-tagged::{sql-specs}/docs/docs.csv-spec[iifWithDefaultValue]
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----
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["source","sql",subs="attributes,callouts,macros"]
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----
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include-tagged::{sql-specs}/docs/docs.csv-spec[iifWithoutDefaultValue]
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----
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[TIP]
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=================
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*IIF* functions can be combined to implement more complex logic simulating the <<sql-functions-conditional-case>>
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expression. E.g.:
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[source, sql]
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IIF(a = 1, 'one', IIF(a = 2, 'two', IIF(a = 3, 'three', 'many')))
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=================
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[[sql-functions-conditional-isnull]]
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==== `ISNULL`
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.Synopsis:
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[source, sql]
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----
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ISNULL(
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expression, <1>
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expression) <2>
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----
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*Input*:
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<1> 1st expression
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<2> 2nd expression
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*Output*: 2nd expression if 1st expression is null, otherwise 1st expression.
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.Description
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Variant of <<sql-functions-conditional-coalesce>> with only two arguments.
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Returns the first of its arguments that is not null.
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If all arguments are null, then it returns `null`.
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["source","sql",subs="attributes,callouts,macros"]
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----
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include-tagged::{sql-specs}/docs/docs.csv-spec[isNullReturnFirst]
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----
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["source","sql",subs="attributes,callouts,macros"]
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----
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include-tagged::{sql-specs}/docs/docs.csv-spec[isNullReturnSecond]
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----
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[[sql-functions-conditional-least]]
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==== `LEAST`
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.Synopsis:
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[source, sql]
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----
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LEAST(
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expression, <1>
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expression, <2>
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...)
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----
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*Input*:
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<1> 1st expression
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<2> 2nd expression
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...
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**N**th expression
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LEAST can take an arbitrary number of arguments and
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all of them must be of the same data type.
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*Output*: one of the expressions or `null`
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.Description
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Returns the argument that has the smallest value which is not null.
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If all arguments are null, then it returns `null`.
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["source","sql",subs="attributes,callouts,macros"]
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----
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include-tagged::{sql-specs}/docs/docs.csv-spec[leastReturnNonNull]
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----
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["source","sql",subs="attributes,callouts,macros"]
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----
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include-tagged::{sql-specs}/docs/docs.csv-spec[leastReturnNull]
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----
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[[sql-functions-conditional-nullif]]
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==== `NULLIF`
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.Synopsis:
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[source, sql]
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----
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NULLIF(
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expression, <1>
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expression) <2>
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----
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*Input*:
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<1> 1st expression
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<2> 2nd expression
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*Output*: `null` if the 2 expressions are equal, otherwise the 1st expression.
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.Description
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Returns `null` when the two input expressions are equal and
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if not, it returns the 1st expression.
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["source","sql",subs="attributes,callouts,macros"]
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----
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include-tagged::{sql-specs}/docs/docs.csv-spec[nullIfReturnFirst]
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----
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["source","sql",subs="attributes,callouts,macros"]
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----
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include-tagged::{sql-specs}/docs/docs.csv-spec[nullIfReturnNull]
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----
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[[sql-functions-conditional-nvl]]
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==== `NVL`
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.Synopsis:
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[source, sql]
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----
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NVL(
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expression, <1>
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expression) <2>
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----
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*Input*:
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<1> 1st expression
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<2> 2nd expression
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*Output*: 2nd expression if 1st expression is null, otherwise 1st expression.
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.Description
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Variant of <<sql-functions-conditional-coalesce>> with only two arguments.
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Returns the first of its arguments that is not null.
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If all arguments are null, then it returns `null`.
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["source","sql",subs="attributes,callouts,macros"]
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----
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include-tagged::{sql-specs}/docs/docs.csv-spec[nvlReturnFirst]
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----
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["source","sql",subs="attributes,callouts,macros"]
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----
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include-tagged::{sql-specs}/docs/docs.csv-spec[nvlReturnSecond]
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----
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