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https://issues.apache.org/jira/browse/DERBY-2374?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12773230#action_12773230
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Mamta A. Satoor commented on DERBY-2374:
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Just wanted to post results of parser behavior when I push the consumption of (
further down in the derived table rules for the query below.
select c1 from ((select c1 from t1) union (select c1 from t2)) t1t2;
For the query above, Derby currently chooses one of the following 2 rules for
the string after the first "from" clause
1) <table reference> = <left paren> <table reference> <right paren>
2) <table reference> = <derived table> [AS] <correlation name>
The decision is based on the fact if there is a "select" clause following ( or
not. If there is (Select combination, then Derby used the 2nd rule. Based on
this, since right after the first "from" clause, we just have ( and not
(select, we choose rule 1). Next, the tokens are (select and we choose rule 2).
The <derived table> leads to consuming (select c1 from t1) and next we look
for [AS] <correlation name> but what we have is UNION and hence parser error. I
changed the rule for <derived table> such that if there is a UNION clause after
the ), then we should continue handling UNION grammar. This was a progress
because now <derived table> can consume (select c1 from t1) union (select c1
from t2). But after this, we look for [AS] <correlation name> again but what we
get is the last ) for the query above. This ) would have been consumed by rule
1 which was applied first but we are never going to get to that point because
we are still working on the <derived table> grammar and that grammar does not
see [AS] <correlation name> and because of that parser throws an exception for
missing correlation name even with my changes. I think what is happeneing is we
are incorrectly identifying the queries around UNION clause to be not derived
tables. Changing that might fix the problem but at this point, I am not sure
how involved that is.
Also, there is following piece of code in the parser for tableFactor
// There is a grammar ambiguity with nested parentheses here.
// A series of left parentheses could introduce either a table
// reference or a derived table. For example:
//
// (((select c from t) a inner join (select d from s) b ))
//
// and:
//
// (((select c from t) a)))
//
// To distinguish these two cases, we consider anything that starts
// with a single parenthesis and either SELECT or VALUES to be
// a derived table, and anything else to be a table reference.
// Note that we can't use the subqueryFollows() lookahead method,
// because it skips over all leading left parentheses to decide
// whether a subquery follows.
LOOKAHEAD( {
getToken(1).kind == LEFT_PAREN &&
(
getToken(2).kind == SELECT ||
getToken(2).kind == VALUES
)
} )
derivedTable = derivedTable() [ <AS> ] correlationName =
identifier(Limits.MAX_IDENTIFIER_LENGTH, true)
[ <LEFT_PAREN> derivedRCL = derivedColumnList() <RIGHT_PAREN> ]
[ optionalTableClauses = optionalTableProperties() ]
I tried writing a query using inner join to see how this code path is used but
couldn't quite come up with a query satisfying the comment
// (((select c from t) a inner join (select d from s) b ))
It will be interesting to see what query are we trying to solve by this special
code as per the comments.
> 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: 10.1.3.3, 10.2.2.0, 10.3.3.1, 10.4.2.1, 10.5.3.1,
> 10.6.0.0
> 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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