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To find the MIN() or MAX() value for a specific indexed column. To sort or group a table (under certain conditions). To optimize queries using only indexes without consulting the data rows. Indexes in a database work like an index in a book.
MySQL CREATE INDEX Example. The SQL statement below creates an index named "idx_lastname" on the "LastName" column in the "Persons" table: CREATE INDEX idx_lastname. ON Persons (LastName); If you want to create an index on a combination of columns, you can list the column names within the parentheses, separated by commas: CREATE INDEX idx_pname.
CREATE INDEX enables you to add indexes to existing tables. CREATE INDEX is mapped to an ALTER TABLE statement to create indexes. See Section 13.1.9, “ALTER TABLE Statement”. CREATE INDEX cannot be used to create a PRIMARY KEY; use ALTER TABLE instead.
10.3.1 How MySQL Uses Indexes. 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. The larger the table, the more this costs. If the table has an index for the columns in question, MySQL can quickly determine ...
CREATE TABLE test ( id INT NOT NULL, last_name CHAR(30) NOT NULL, first_name CHAR(30) NOT NULL, PRIMARY KEY (id), INDEX name (last_name,first_name) ); The name index is an index over the last_name and first_name columns.
To create an index for a column or a list of columns, you specify the index name, the table to which the index belongs, and the column list. For example, to add a new index for the column c4, you use the following statement:
4 sty 2023 · CREATE INDEX names ON employees (last_name, first_name); In this case, the CREATE INDEX statement differs slightly. Now in the parentheses after the table name (employees), two columns are listed: last_name and then first_name. This creates a multi-column index on both columns.
