What does MUL mean in MySQL?

This article explains the meaning of the MUL index in MySQL. MUL indicates that a column can have non-unique values and is commonly used with foreign keys.

In MySQL, MUL appears in the index information when a column is indexed but is not unique (UNIQUE). This means that the column can have duplicate values. MUL often shows up when a column is part of a foreign key.

Detailed explanation:

• In MySQL, MUL in the table index information indicates that a column can contain duplicate values and is indexed to improve search performance.
• When a column is part of a foreign key, MySQL automatically creates an index for that column, and if the column can have multiple identical values, it will be marked as MUL (short for "Multiple").

For example: If you create a foreign key linking to another table, the column containing the foreign key will often be marked as MUL because its values may not be unique, but it still needs to be indexed to improve query performance.

System requirements:

• MySQL 5.7 or above.

Tips:

• Use MUL when your column may contain multiple identical values and you need to improve query performance through indexing.

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