Preventing Duplicates from Occurring in a Table
Problem
You want to prevent a table from ever containing duplicates.
Solution
Use a PRIMARY
KEY
or a
UNIQUE
index.
Discussion
To make sure that rows in a table are unique, some column or
combination of columns must be required to contain unique values in
each row. When this requirement is satisfied, you can refer to any row
in the table unambiguously by using its unique identifier. To make
sure a table has this characteristic, include a PRIMARY
KEY
or UNIQUE
index in the table structure when you
create the table. The following table contains no such index, so it
would allow duplicate rows:
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
does not allow 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. Dealing with Duplicates When Loading Rows into a Table 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 ...
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