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94 lines
2.2 KiB
Plaintext
94 lines
2.2 KiB
Plaintext
SQL Language Extension: derived tables
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Function:
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A derived table is a table derived from a select statement.
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Derived tables can also be nested to build complex queries.
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They can be joined the same as normal tables and views.
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Author:
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Arno Brinkman <firebird@abvisie.nl>
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Format:
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SELECT
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<select list>
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FROM
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<table reference list>
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<table reference list> ::= <table reference> [{<comma> <table reference>}...]
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<table reference> ::=
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<table primary>
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| <joined table>
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<table primary> ::=
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<table> [[AS] <correlation name>]
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| <derived table>
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<derived table> ::=
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<query expression> [[AS] <correlation name>]
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[<left paren> <derived column list> <right paren>]
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<derived column list> ::= <column name> [{<comma> <column name>}...]
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Notes:
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Every column in the derived table must have a name. Unnamed expressions like
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constants should be added with an alias or the column list should be used.
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The number of columns in the column list should be the same as the number of
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columns from the query expression.
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The optimizer can handle a derived table very efficiently, but if the
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derived table contains a sub-select then no join order can be made (if the
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derived table is included in an inner join).
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Examples:
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a) Simple derived table:
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SELECT
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*
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FROM
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(SELECT
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RDB$RELATION_NAME, RDB$RELATION_ID
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FROM
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RDB$RELATIONS) AS R (RELATION_NAME, RELATION_ID)
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b) Aggregate on a derived table which also contains an aggregate
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SELECT
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DT.FIELDS,
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Count(*)
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FROM
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(SELECT
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R.RDB$RELATION_NAME,
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Count(*)
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FROM
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RDB$RELATIONS R
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JOIN RDB$RELATION_FIELDS RF ON (RF.RDB$RELATION_NAME = R.RDB$RELATION_NAME)
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GROUP BY
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R.RDB$RELATION_NAME) AS DT (RELATION_NAME, FIELDS)
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GROUP BY
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DT.FIELDS
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c) UNION and ORDER BY example:
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SELECT
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DT.*
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FROM
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(SELECT
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R.RDB$RELATION_NAME,
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R.RDB$RELATION_ID
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FROM
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RDB$RELATIONS R
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UNION ALL
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SELECT
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R.RDB$OWNER_NAME,
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R.RDB$RELATION_ID
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FROM
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RDB$RELATIONS R
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ORDER BY
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2) AS DT
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WHERE
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DT.RDB$RELATION_ID <= 4
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