Indexes are used to find rows with specific column values quickly. Without an index, MySQL must begin with the first row and then read through the entire table to find the relevant rows. Five Types of Indexes When you create an index or add one to an existing table, you’ll create it as one of several types of indexes. A unique index is one in which all column values must be unique. In a single column unique index there can be no duplication of values in the column being indexed. In a multi-column unique index the values can be duplicated in a single column, but the combination of column values in each row must be unique. You use a unique index to prevent duplicate values and you often define the index after a table has been created. A primary key is a unique index in which no value can be NULL. Every row must have a value for the column or combination of columns. You would usually define a primary key on the smallest number of columns possible because of this, and most of the time a