In InnoDB, every 'secondary index' includes a copy of the columns of the PRIMARY KEY. Therefore keys should be as small as possible (especially regarding composed keys). Larger indexes require more disk space, memory, and I/O. With the help of the primary key, one can find each row uniquely. Note that the columns of the primary key should be specified in logical sort order, which may be different from the order in which the columns were defined, as in the example above. A primary key is an attribute to make a column or a set of columns accept only unique values. For example: CREATE TABLE invoice_line_items (įOREIGN KEY (InvoiceNum) REFERENCES - references to an attribute of a table Inline syntax is not permitted here, as only one column may be declared PRIMARY KEY inline. A multi-column primary key is defined by listing the participating columns in a separate PRIMARY KEY clause. on the child table of a foreign-key relationship. It is also possible to define a primary key comprising more than one column. This form of the command is shorter and easier to read. PersonID INT UNSIGNED NOT NULL PRIMARY KEY, If the primary key consists of a single column, the PRIMARY KEY clause can be placed inline with the column definition: CREATE TABLE Person ( Defining one column as Primary Key (inline definition) However, despite its name, it is not its purpose to guarantee that values are incremental, merely that they are sequential and unique.Īn auto-increment INT value will not reset to its default start value if all rows in the table are deleted, unless the table is truncated using TRUNCATE TABLE statement. This value will (normally) increase by 1 whenever a new record is added, starting from a default value of 1. Often, an AUTO_INCREMENT INT also known as "surrogate key", is used for thin index optimization and relations with other tables. InnoDB will automatically create one in its absence, (as seen in MySQL documentation) though this is less desirable. An index is created, and if not explicitly declared as NOT NULL, MySQL will declare them so silently and implicitly.Ī table can have only one PRIMARY KEY, and each table is recommended to have one.
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