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-> WHERE aAircraftTypeID = atAircraftTypeID -> GROUP BY aAircraftTypeID) -> AS sq; +---------------+--------------+ | MAX(sqcount) | AircraftName | +---------------+--------------+ | 6 | Boeing 747 | +---------------+--------------+ 1 row in set (001 sec)
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Notice that, in this case, the result set generated by the inner query is stored in a temporary table and used in the FROM clause of the outer query Such a table is referred to as a derived table or a materialized subquery Notice also that when using subquery results in this manner, the derived table must be first aliased to a table name or else MySQL will not know how to refer to fields within it As an example, look what happens if you re-run the previous query without the table alias:
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mysql> SELECT MAX(sqcount), sqAircraftName FROM -> (SELECT COUNT(aAircraftID) AS count, atAircraftName -> FROM aircraft AS a, aircrafttype AS at -> WHERE aAircraftTypeID = atAircraftTypeID -> GROUP BY aAircraftTypeID); ERROR 1248 (42000): Every derived table must have its own alias
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Another example might involve finding out on which days of the week is the number of flights operated by the airline above average Here, too, a subquery can be used to generate a table containing a count of the number of flights on each day, and this table can then be used (within the outer query s FROM clause) to compare each day s count with the average value:
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mysql> SELECT xDepDay FROM -> (SELECT fdDepDay, COUNT(fdFlightID) AS c -> FROM flightdep AS fd -> GROUP BY fdDepDay) -> AS x -> WHERE xc > -> (SELECT COUNT(fdFlightID)/7 FROM flightdep AS fd); +--------+ | DepDay | +--------+ | 1 | | 2 | | 3 | | 4 | | 5 | +--------+ 5 rows in set (000 sec)
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Subqueries and Other DML Statements
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The examples you ve seen thus far have only used subqueries in the context of a SELECT statement However, subqueries can just as easily be used to constrain UPDATE and DELETE statements Here s an example that deletes all routes originating from Changi Airport:
mysql> DELETE FROM route -> WHERE routeFrom = -> (SELECT AirportID FROM airport -> WHERE AirportCode = 'SIN'); Query OK, 3 rows affected (000 sec)
The IN membership test works here, too consider the next example, which deletes all routes originating in the United Kingdom:
mysql> DELETE FROM route -> WHERE routeFrom IN -> (SELECT AirportID FROM airport -> WHERE CountryCode = 'UK'); Query OK, 5 rows affected (005 sec)
UPDATEs can be performed in a similar manner Consider the following query, which turns all Boeing aircraft into Airbus A330 aircraft:
mysql> UPDATE aircraft -> SET AircraftTypeID = -> (SELECT AircraftTypeID -> FROM aircrafttype -> WHERE AircraftName = 'Airbus A330') -> WHERE AircraftTypeID IN -> (SELECT AircraftTypeID -> FROM aircrafttype -> WHERE AircraftName LIKE 'Boeing%'); Query OK, 5 rows affected (001 sec) Rows matched: 5 Changed: 5 Warnings: 0
Another example might involve reading flight departure times from the flightdep table and writing them to the flight table, using the flight number as link Here s how:
mysql> ALTER TABLE flight ADD DepTime TIME NOT NULL; Query OK, 32 rows affected (005 sec) Records: 32 Duplicates: 0 Warnings: 0 mysql> UPDATE flight SET DepTime = -> (SELECT DepTime FROM flightdep -> WHERE flightdepFlightID = flightFlightID -> GROUP BY flightdepFlightID); Query OK, 32 rows affected (002 sec) Rows matched: 32 Changed: 32 Warnings: 0
4:
U s i n g J o i n s , S u b q u e r i e s , a n d Vi e w s
Circular References in UPDATE and DELETE Statements MySQL won t let you delete or update a table s data if you re simultaneously reading that same data with a subquery, as doing so raises the possibility that your subquery might reference rows that have already been deleted or altered Therefore, the table named in an outer DELETE or UPDATE DML statement cannot appear in the FROM clause of an inner subquery To illustrate this, consider the situation where the airline needs to remove orphan routes routes without a corresponding flight from the database This appears simple at first glance: Find these routes using a LEFT JOIN between the route and flight tables with an IS NULL clause and then delete them using a subquery Here s the query:
mysql> DELETE FROM route -> WHERE RouteID IN -> (SELECT rRouteID -> FROM route AS r -> LEFT JOIN flight AS f -> USING (RouteID) -> WHERE fFlightID IS NULL); ERROR 1093 (HY000): You can't specify target table 'route' for update in FROM clause
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