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https://issues.apache.org/jira/browse/DERBY-2374?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12769304#action_12769304
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Dag H. Wanvik commented on DERBY-2374:
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Just to make sure it is allowed, I did the derivation as an exercise ;-)
Quote from std (2003):
<query expression body> ::=
<query term>
| <query expression body> UNION [ ALL | DISTINCT ]
[ <corresponding spec> ] <query term>
| <query expression body> EXCEPT [ ALL | DISTINCT ]
[ <corresponding spec> ] <query term>
<query term> ::=
<query primary>
| <query term> INTERSECT [ ALL | DISTINCT ]
[ <corresponding spec> ] <query primary>
<query primary> ::=
<simple table>
| <left paren> <query expression body> <right paren>
<simple table> ::=
<query specification>
| <table value constructor>
| <explicit table>
<query specification> ::=
SELECT [ <set quantifier> ] <select list> <table expression>
Example:
SELECT c1 FROM ((SELECT c1 FROM t1) UNION (SELECT c1 FROM t2)) t1t2;
Derivation:
<query expression body>
<query expression body> UNION ...
<query term> UNION ...
<query primary> UNION ...
<left paren> <query expression body> <right paren> UNION ...
( <query expression body> ) UNION ...
( <query term> ) UNION ...
( <query primary> ) UNION ...
( <simple table> ) UNION ...
( <query specification> ) UNION ...
( SELECT c1 FROM t1 ) UNION ...
Similarly for the right hand side of the union.
> UNION PROBLEM WITH PARENTHESIS
> ------------------------------
>
> Key: DERBY-2374
> URL: https://issues.apache.org/jira/browse/DERBY-2374
> Project: Derby
> Issue Type: Bug
> Components: SQL
> Affects Versions: 10.2.2.0
> Reporter: Kenneth Gee
> Priority: Minor
>
> The following query shows the error using the Derby demo toursDB:
> SELECT FLIGHT_ID , SEGMENT_NUMBER , ARRIVE_TIME
> FROM (
> (SELECT HW.FLIGHT_ID , HW.SEGMENT_NUMBER , HW.ARRIVE_TIME FROM
> FLIGHTS HW)
> UNION
> (SELECT SW.FLIGHT_ID , SW.SEGMENT_NUMBER , SW.ARRIVE_TIME FROM
> FLIGHTS SW)
> ) SRC;
> ERROR 42X01: Syntax error: Encountered "UNION" at line 4, column 12.
> The following query works:
> SELECT FLIGHT_ID , SEGMENT_NUMBER , ARRIVE_TIME
> FROM (
> SELECT HW.FLIGHT_ID , HW.SEGMENT_NUMBER , HW.ARRIVE_TIME FROM
> FLIGHTS HW
> UNION
> SELECT SW.FLIGHT_ID , SW.SEGMENT_NUMBER , SW.ARRIVE_TIME FROM
> FLIGHTS SW
> ) SRC;
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