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68 lines
2.2 KiB
Plaintext
68 lines
2.2 KiB
Plaintext
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Select statements and select expressions
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Semantics:
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A select statement is used to return data to the caller (PSQL module or the
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client program). Select expressions retrieve parts of data that construct both
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the final the result set or any of the intermediate sets. Select expressions
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are also known as subqueries.
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Syntax rules:
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<select statement> ::=
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<select expression> [FOR UPDATE] [WITH LOCK]
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<select expression> ::=
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<query specification> [UNION [{ALL | DISTINCT}] <query specification>]
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<query specification> ::=
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SELECT [FIRST <value>] [SKIP <value>] <select list>
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FROM <table expression list>
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WHERE <search condition>
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GROUP BY <group value list>
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HAVING <group condition>
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PLAN <plan item list>
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ORDER BY <sort value list>
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ROWS <value> [TO <value>]
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<table expression> ::=
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<table name> | <joined table> | <derived table>
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<joined table> ::=
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{<cross join> | <qualified join>}
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<cross join> ::=
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<table expression> CROSS JOIN <table expression>
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<qualified join> ::=
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<table expression> [{INNER | {LEFT | RIGHT | FULL} [OUTER]}] JOIN <table expression>
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ON <join condition>
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<derived table> ::=
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'(' <select expression> ')'
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Conclusions:
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* FOR UPDATE mode and row locking can only be performed for a final dataset,
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they cannot be applied to a subquery
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* Unions are allowed inside any subquery
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* Clauses FIRST, SKIP, PLAN, ORDER BY, ROWS are allowed for any subquery
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Notes:
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* Either FIRST/SKIP or ROWS is allowed, they cannot be mixed together
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(a syntax error is thrown).
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* INSERT statement accepts a select expression to create a dataset to be
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inserted into a table. So its SELECT part supports all the features defined
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above.
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* UPDATE and DELETE statements are always based on an implicit cursor
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iterating through its target table and limited with the WHERE clause. You
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may also specify the final parts of the select expression syntax to limit
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the number of affected rows or optimize the statement. The following clauses
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are allowed at the end of the UPDATE/DELETE statements: PLAN, ORDER BY and
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ROWS.
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