Database - index

Create index
Indexes are created in existing tables , It makes the positioning of rows faster and more efficient . You can create indexes on one or more columns of a table , Each index will be named . The user cannot see the index , They can only be used to speed up queries .

notes ： Updating a table with an index takes more time than updating a table without an index , This is because the index itself needs to be updated . therefore , It is ideal to create an index only on the columns that are often used for searching .

Unique index (Unique Index)
Create a unique index on the table . A unique index means that two rows cannot have the same index value .

CREATE UNIQUE INDEX The index name
ON The name of the table ( Column name )
“ Column name ” Specify the columns you want to index .

Indexes

Simple index
Create a simple index on the table . When we omit keywords UNIQUE when , You can use duplicate values .

CREATE INDEX The index name
ON The name of the table ( Column name )
“ Column name ” Specify the columns you want to index .

example
This example creates a simple index , be known as “PersonIndex”, stay Person Tabular LastName Field ：

CREATE INDEX PersonIndex
ON Person (LastName)
If you want to index the values in a column in descending order , You can add a reserved word after the column name DESC：

CREATE INDEX PersonIndex
ON Person (LastName DESC)
If you want to index more than one column , You can list the names of these columns in parentheses , Separated by commas ：

CREATE INDEX PersonIndex
ON Person (LastName, FirstName)