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(mysql.info) safe-updates

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 8.5.4.2 Using the `--safe-updates' Option
 .........................................
 
 For beginners, a useful startup option is -safe-updates (or
 -i-am-a-dummy, which has the same effect).  It is helpful for cases
 when you might have issued a `DELETE FROM TBL_NAME' statement but
 forgotten the `WHERE' clause. Normally, such a statement deletes all
 rows from the table. With -safe-updates, you can delete rows only by
 specifying the key values that identify them. This helps prevent
 accidents.
 
 When you use the -safe-updates option, `mysql' issues the following
 statement when it connects to the MySQL server:
 
      SET SQL_SAFE_UPDATES=1,SQL_SELECT_LIMIT=1000, SQL_MAX_JOIN_SIZE=1000000;
 
 See  set-option.
 
 The `SET' statement has the following effects:
 
    * You are not allowed to execute an `UPDATE' or `DELETE' statement
      unless you specify a key constraint in the `WHERE' clause or
      provide a `LIMIT' clause (or both). For example:
 
           UPDATE TBL_NAME SET NOT_KEY_COLUMN=VAL WHERE KEY_COLUMN=VAL;
 
           UPDATE TBL_NAME SET NOT_KEY_COLUMN=VAL LIMIT 1;
 
    * The server limits all large `SELECT' results to 1,000 rows unless
      the statement includes a `LIMIT' clause.
 
    * The server aborts multiple-table `SELECT' statements that probably
      need to examine more than 1,000,000 row combinations.
 
 To specify limits different from 1,000 and 1,000,000, you can override
 the defaults by using the -select_limit and -max_join_size options:
 
      shell> mysql --safe-updates --select_limit=500 --max_join_size=10000
 
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