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SQL: Implement CASE... WHEN... THEN... ELSE... END (#41349)
Implement the ANSI SQL CASE expression which provides the if/else functionality common to most programming languages. The CASE expression can have multiple WHEN branches and becomes a powerful tool for SQL queries as it can be used in SELECT, WHERE, GROUP BY, HAVING and ORDER BY clauses. Closes: #36200 (cherry picked from commit 8b25774)
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‎docs/reference/sql/functions/conditional.asciidoc

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[role="xpack"]
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[testenv="basic"]
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[[sql-functions-conditional]]
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=== Conditional Functions
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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-coalesce]]
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==== `COALESCE`
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‎docs/reference/sql/functions/index.asciidoc

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** <<sql-functions-type-conversion-cast>>
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** <<sql-functions-type-conversion-convert>>
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* <<sql-functions-conditional>>
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** <<sql-functions-conditional-case>>
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** <<sql-functions-conditional-coalesce>>
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** <<sql-functions-conditional-greatest>>
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** <<sql-functions-conditional-ifnull>>

‎x-pack/plugin/sql/qa/src/main/resources/command.csv-spec

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@@ -25,6 +25,7 @@ STDDEV_POP |AGGREGATE
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SUM_OF_SQUARES |AGGREGATE
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VAR_POP |AGGREGATE
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HISTOGRAM |GROUPING
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CASE |CONDITIONAL
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COALESCE |CONDITIONAL
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GREATEST |CONDITIONAL
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IFNULL |CONDITIONAL

‎x-pack/plugin/sql/qa/src/main/resources/conditionals.csv-spec

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caseField
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SELECT emp_no, CASE WHEN emp_no - 10000 < 10 THEN 'First 10' ELSE 'Second 10' END as "case" FROM test_emp WHERE emp_no >= 10005
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ORDER BY emp_no LIMIT 10;
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emp_no | case
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--------+-----------
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10005 | First 10
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10006 | First 10
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10007 | First 10
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10008 | First 10
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10009 | First 10
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10010 | Second 10
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10011 | Second 10
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10012 | Second 10
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10013 | Second 10
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10014 | Second 10
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;
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caseFieldWithoutAlias
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SELECT emp_no, CASE WHEN emp_no - 10000 < 10 THEN emp_no ELSE emp_no % 10 END FROM test_emp WHERE emp_no >= 10005
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ORDER BY emp_no LIMIT 10;
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emp_no | CASE WHEN emp_no - 10000 < 10 THEN emp_no ELSE emp_no % 10 END
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--------+----------------------------------------------------------------
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10005 | 10005
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10006 | 10006
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10007 | 10007
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10008 | 10008
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10009 | 10009
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10010 | 0
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10011 | 1
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10012 | 2
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10013 | 3
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10014 | 4
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;
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caseFieldNoElse
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SELECT emp_no, CASE WHEN emp_no - 10000 < 10 THEN 'First 10' END as "case" FROM test_emp WHERE emp_no >= 10005
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ORDER BY emp_no LIMIT 10;
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emp_no | case
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--------+----------
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10005 | First 10
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10006 | First 10
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10007 | First 10
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10008 | First 10
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10009 | First 10
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10010 | null
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10011 | null
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10012 | null
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10013 | null
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10014 | null
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;
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caseWhere
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SELECT last_name FROM test_emp WHERE CASE WHEN LENGTH(last_name) < 7 THEN 'ShortName' ELSE 'LongName' END = 'LongName'
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ORDER BY emp_no LIMIT 10;
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last_name
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-----------
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Facello
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Bamford
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Koblick
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Maliniak
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Preusig
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Zielinski
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Kalloufi
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Piveteau
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Bridgland
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Nooteboom
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;
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caseWhereNoElse
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SELECT last_name FROM test_emp WHERE CASE WHEN LENGTH(last_name) < 7 THEN 'ShortName' END IS NOT NULL
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ORDER BY emp_no LIMIT 10;
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last_name
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-----------
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Simmel
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Peac
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Sluis
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Terkki
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Genin
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Peha
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Erde
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Famili
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Pettey
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Heyers
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;
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caseOrderBy
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schema::last_name:s|languages:byte|emp_no:i
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SELECT last_name, languages, emp_no FROM test_emp WHERE emp_no BETWEEN 10005 AND 10015
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ORDER BY CASE WHEN languages >= 3 THEN 'first' ELSE 'second' END, emp_no LIMIT 10;
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last_name | languages | emp_no
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-----------+-----------+--------
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Preusig | 3 | 10006
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Zielinski | 4 | 10007
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Piveteau | 4 | 10010
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Sluis | 5 | 10011
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Bridgland | 5 | 10012
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Genin | 5 | 10014
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Nooteboom | 5 | 10015
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Maliniak | 1 | 10005
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Kalloufi | 2 | 10008
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Peac | 1 | 10009
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;
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caseOrderByNoElse
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schema::last_name:s|languages:byte|emp_no:i
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SELECT last_name, languages, emp_no FROM test_emp WHERE emp_no BETWEEN 10005 AND 10015
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ORDER BY CASE WHEN languages >= 3 THEN 'first' END NULLS FIRST, emp_no LIMIT 10;
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last_name | languages | emp_no
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-----------+-----------+--------
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Maliniak | 1 | 10005
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Kalloufi | 2 | 10008
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Peac | 1 | 10009
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Terkki | 1 | 10013
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Preusig | 3 | 10006
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Zielinski | 4 | 10007
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Piveteau | 4 | 10010
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Sluis | 5 | 10011
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Bridgland | 5 | 10012
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Genin | 5 | 10014
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;
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caseGroupBy
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schema::count:l|lang_skills:s
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SELECT count(*) AS count, CASE WHEN NVL(languages, 0) <= 1 THEN 'zero-to-one' ELSE 'multilingual' END as lang_skills
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FROM test_emp GROUP BY lang_skills ORDER BY lang_skills;
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count | lang_skills
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-------+--------------
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75 | multilingual
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25 | zero-to-one
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;
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caseGroupByNoElse
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schema::count:l|lang_skills:s
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SELECT count(*) AS count, CASE WHEN NVL(languages, 0) <= 1 THEN 'zero-to-one' END as lang_skills
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FROM test_emp GROUP BY lang_skills ORDER BY lang_skills;
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count | lang_skills
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-------+-------------
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75 | null
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25 | zero-to-one
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;
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caseGroupByComplexNested
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schema::count:l|lang_skills:s
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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 FROM test_emp GROUP BY lang_skills ORDER BY 2;
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count | lang_skills
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-------+--------------
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19 | bilingual
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39 | multilingual
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15 | one
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17 | trilingual
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10 | zero
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;
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caseGroupByAndHaving
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schema::count:l|gender:s|languages:byte
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SELECT count(*) AS count, gender, languages FROM test_emp
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GROUP BY 2, 3 HAVING CASE WHEN count(*) > 10 THEN 'many' ELSE 'a few' END = 'many'
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ORDER BY 2, 3;
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count | gender | languages
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----------+-------------+---------------
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11 | M | 2
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11 | M | 3
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11 | M | 4
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;

‎x-pack/plugin/sql/qa/src/main/resources/docs/docs.csv-spec

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@@ -202,6 +202,7 @@ STDDEV_POP |AGGREGATE
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SUM_OF_SQUARES |AGGREGATE
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VAR_POP |AGGREGATE
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HISTOGRAM |GROUPING
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CASE |CONDITIONAL
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COALESCE |CONDITIONAL
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GREATEST |CONDITIONAL
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IFNULL |CONDITIONAL
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///////////////////////////////
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//
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// Null handling
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// Conditional
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//
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///////////////////////////////
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case
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schema::case:s
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// tag::case
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SELECT CASE WHEN 1 > 2 THEN 'elastic'
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WHEN 2 <= 3 THEN 'search'
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END AS "case";
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case
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---------------
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search
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// end::case
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;
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caseReturnNull
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schema::case:s
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// tag::caseReturnNull
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SELECT CASE WHEN 1 > 2 THEN 'elastic'
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WHEN 2 > 10 THEN 'search'
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END AS "case";
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case
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---------------
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null
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// end::caseReturnNull
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;
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caseWithElse
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schema::case:s
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// tag::caseWithElse
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SELECT CASE WHEN 1 > 2 THEN 'elastic'
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WHEN 2 > 10 THEN 'search'
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ELSE 'default'
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END AS "case";
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case
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---------------
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default
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// end::caseWithElse
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;
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caseWithOperand
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schema::case:s
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// tag::caseWithOperand
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SELECT CASE 5
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WHEN 1 THEN 'elastic'
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WHEN 2 THEN 'search'
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WHEN 5 THEN 'elasticsearch'
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END AS "case";
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case
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---------------
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elasticsearch
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// end::caseWithOperand
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;
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caseWithOperandAndElse
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schema::case:s
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// tag::caseWithOperandAndElse
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SELECT CASE 5
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WHEN 1 THEN 'elastic'
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WHEN 2 THEN 'search'
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WHEN 3 THEN 'elasticsearch'
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ELSE 'default'
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END AS "case";
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case
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---------------
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default
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// end::caseWithOperandAndElse
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;
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coalesceReturnNonNull
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// tag::coalesceReturnNonNull
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SELECT COALESCE(null, 'elastic', 'search') AS "coalesce";

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