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Dmitry Yemanov 2020-04-06 11:32:31 +03:00
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31
doc/sql.extensions/README.derived_tables.txt
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@ -23,6 +23,7 @@ Format:
<table primary> ::= <table primary> ::=
<table> [[AS] <correlation name>] <table> [[AS] <correlation name>]
| <derived table> | <derived table>
| <lateral derived table>
<derived table> ::= <derived table> ::=
<query expression> [[AS] <correlation name>] <query expression> [[AS] <correlation name>]
@ -30,14 +31,18 @@ Format:
<derived column list> ::= <column name> [{<comma> <column name>}...] <derived column list> ::= <column name> [{<comma> <column name>}...]
<lateral derived table> ::= LATERAL <derived table>
Notes: Notes:
Every column in the derived table must have a name. Unnamed expressions like - Every column in the derived table must have a name. Unnamed expressions like
constants should be added with an alias or the column list should be used. constants should be added with an alias or the column list should be used.
The number of columns in the column list should be the same as the number of - The number of columns in the column list should be the same as the number of
columns from the query expression. columns from the query expression.
The optimizer can handle a derived table very efficiently, but if the - The optimizer can handle a derived table very efficiently, but if the
derived table contains a sub-select then no join order can be made (if the derived table contains a sub-select then no join order can be made (if the
derived table is included in an inner join). derived table is included in an inner join).
- Keyword LATERAL allows the derived table to reference fields from the priorly
listed tables in the current <table reference list>.
Examples: Examples:
@ -52,7 +57,7 @@ a) Simple derived table:
RDB$RELATIONS) AS R (RELATION_NAME, RELATION_ID) RDB$RELATIONS) AS R (RELATION_NAME, RELATION_ID)
b) Aggregate on a derived table which also contains an aggregate b) Aggregate on a derived table which also contains an aggregate:
SELECT SELECT
DT.FIELDS, DT.FIELDS,
@ -91,3 +96,13 @@ c) UNION and ORDER BY example:
WHERE WHERE
DT.RDB$RELATION_ID <= 4 DT.RDB$RELATION_ID <= 4
d) LATERAL derived table:
SELECT
*
FROM
(SELECT RDB$RELATION_NAME, RDB$RELATION_ID FROM RDB$RELATIONS)
AS R (RELATION_NAME, RELATION_ID)
CROSS JOIN LATERAL
(SELECT COUNT(*) FROM RDB$RELATION_FIELDS WHERE RDB$RELATION_NAME = R.RELATION_NAME)
AS RF (FIELD_COUNT)