Handling Duplicates - MySQL - page 551

CREATE TABLE person
(
last_name CHAR ( 20 ),
first_name CHAR ( 20 ),
address CHAR ( 40 )
);
To prevent multiple rows with the same first and last name values from being created
in this table, add a PRIMARY KEY to its definition. When you do this, the indexed columns
must be NOT NULL , because a PRIMARY KEY prohibits NULL values:
CREATE TABLE person
(
last_name CHAR ( 20 ) NOT NULL ,
first_name CHAR ( 20 ) NOT NULL ,
address CHAR ( 40 ),
PRIMARY KEY ( last_name , first_name )
);
The presence of a unique index in a table normally causes an error to occur if you insert
a row into the table that duplicates an existing row in the column or columns that define
the index. Recipe 16.2 discusses how to handle such errors or modify MySQL's duplicate-
handling behavior.
Another way to enforce uniqueness is to add a UNIQUE index rather than a PRIMARY KEY
to a table. The two types of indexes are similar, but a UNIQUE index can be created on
columns that permit NULL values. For the person table, it's likely that you'd require both
the first and last names to be filled in. If so, you still declare the columns as NOT NULL ,
and the following table definition is effectively equivalent to the preceding one:
CREATE TABLE person
(
last_name CHAR ( 20 ) NOT NULL ,
first_name CHAR ( 20 ) NOT NULL ,
address CHAR ( 40 ),
UNIQUE ( last_name , first_name )
);
If a UNIQUE index does happen to permit NULL values, NULL is special because it is the
one value that can occur multiple times. The rationale for this is that it is not possible
to know whether one unknown value is the same as another, so multiple unknown values
are permitted.
Of course, you might want the person table to reflect the real world, in which people
do sometimes have the same name. In this case, you cannot set up a unique index based
on the name columns, because duplicate names must be permitted. Instead, each person
must be assigned some sort of unique identifier, which becomes the value that distin‐
guishes one row from another. In MySQL, it's common to accomplish this by using an
AUTO_INCREMENT column: