Fixed outdated statements on NULL indexing and multi-table delete.

mysql> SELECT * FROM my_table WHERE phone = "";

@end example

In MySQL, as in many other SQL servers, you can't index

columns that can have @code{NULL} values.  You must declare such columns

@code{NOT NULL}. Conversely, you cannot insert @code{NULL} into an indexed

column.

Note that you can only add an index on a column that can have @code{NULL}

values if you are using MySQL Version 3.23.2 or newer and are using the

@code{MyISAM} or @code{InnoDB} table type.

In earlier versions and with other table types, you must declare such

columns @code{NOT NULL}. This also means you cannot then insert

@code{NULL} into an indexed column.

@findex LOAD DATA INFILE

When reading data with @code{LOAD DATA INFILE}, empty columns are updated

@cindex rows, deleting

@cindex tables, deleting rows

As MySQL doesn't support sub-selects or use of more than one table

in the @code{DELETE} statement, you should use the following approach to

delete rows from 2 related tables:

As MySQL doesn't yet support sub-selects, nor the use of more than one

table in the @code{DELETE} statement (prior to Version 4.0), you should

use the following approach to delete rows from 2 related tables:

@enumerate

@item