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Automatic Key Updates and Deletions Foreign keys can certainly take care of ensuring the integrity of newly inserted records But what if a record is deleted from the table named in the REFERENCES clause What happens to all the records in subordinate tables that use this value as a foreign key Obviously, those records should be deleted as well, or else you ll have orphan records cluttering your database MySQL 32350 and later simplifies this task by enabling you to add an ON DELETE clause to the FOREIGN KEYREFERENCES modifier, which tells the database what to do with the orphaned records in such a situation Here s a sequence that demonstrates this:
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mysql> CREATE TABLE aircraft ( -> AircraftID smallint(4) unsigned NOT NULL AUTO_INCREMENT, -> AircraftTypeID smallint(4) unsigned NOT NULL, -> RegNum char(6) NOT NULL, -> LastMaintEnd date NOT NULL, -> NextMaintBegin date NOT NULL, -> NextMaintEnd date NOT NULL, -> PRIMARY KEY (AircraftID), -> UNIQUE RegNum (RegNum), -> FOREIGN KEY (AircraftTypeID) -> REFERENCES aircrafttype (AircraftTypeID) -> ON DELETE CASCADE -> ) ENGINE=INNODB; Query OK, 0 rows affected (017 sec) mysql> INSERT INTO aircraft -> (AircraftID, AircraftTypeID, RegNum, -> LastMaintEnd, NextMaintBegin, NextMaintEnd) -> VALUES -> (3451, 503, 'ZX6488', -> '2007-10-01', '2008-10-23', '2008-10-31'); Query OK, 1 row affected (005 sec) mysql> DELETE FROM aircrafttype; Query OK, 1 row affected (006 sec) mysql> SELECT * FROM aircraft; Empty set (001 sec)
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MySQL 408 and later also lets you perform these automatic actions on updates by allowing the use of an ON UPDATE clause, which works in a similar manner to the ON DELETE clause So, for example, adding the ON UPDATE CASCADE clause to a foreign key definition tells MySQL that when a record is updated in the primary table (the table referenced for foreign key checks), all records using that foreign key value in the current table should also be automatically updated with the new values to ensure the consistency of the system Table 3-1 lists the four keywords that can follow an ON DELETE or ON UPDATE clause
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What It Means Delete all records containing references to the deleted key value
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PARTIII PART PART
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Modify all records containing references to the deleted key value to instead use a NULL value (this can only be used for fields previously marked as NOT NULL) Reject the deletion request until all subordinate records using the deleted key value have themselves been manually deleted and no references exist (this is the default setting, and it s also the safest) Do nothing
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NO ACTION
Table 3-1
Actions Available in ON DELETE and ON UPDATE Clause
C aution Be aware that setting up MySQL for automatic operations through ON UPDATE and
ON DELETE rules can result in serious data corruption if your key relationships aren t set up perfectly For example, if you have a series of tables linked together by foreign key relationships and ON DELETE CASCADE rules, a change in any of the master tables can result in records, even records linked only peripherally to the original deletion, getting wiped out with no warning For this reason, you should check (and then double-check) these rules before finalizing them
Using Indexes
To speed up searches and reduce query execution time, MySQL lets you index particular fields of a table The term index here means much the same as in the real world Similar in concept to the index you find at the end of a book, an index is a list of sorted field values used to simplify the task of locating specific records in response to queries In the absence of an index, MySQL needs to scan each row of the table to find the records matching a particular query This might not cause a noticeable slowdown in smaller tables, but, as table size increases, a complete table scan can add many seconds of overhead to a query An index speeds up things significantly: With an index, MySQL can bypass the full table scan altogether by instead looking up the index and jumping to the appropriate location(s) in the table When looking for records that match a specific search condition, reading an index is typically faster than scanning an entire table This is because indexes are smaller in size and can be searched faster That said, an index does have two important disadvantages: It takes up additional space on disk, and it can affect the speed of INSERT, UPDATE, and DELETE queries because the index must be updated every time table records are added, updated, or deleted Most of the time, though, these reasons shouldn t stop you from using indexes: Disk storage is getting cheaper every day, and MySQL includes numerous optimization techniques to reduce the time spent on updating indexes and searching them for specific values
Part I:
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