MySQL index
MySQL index
The establishment of MySQL index is very important for the efficient operation of MySQL. The index can greatly improve the retrieval of MySQL. speed.
For example, if MySQL that is properly designed and uses indexes is a Lamborghini, then MySQL that is not designed and uses indexes is a human tricycle.
Indexes are divided into single column indexes and combined indexes. A single-column index means that an index only contains a single column. A table can have multiple single-column indexes, but this is not a combined index. Combined index, that is, an index contains multiple columns.
When creating an index, you need to ensure that the index is a condition applied to the SQL query statement (usually as a condition of the WHERE clause).
In fact, the index is also a table, which saves the primary key and index fields and points to the records of the entity table.
The above all talk about the benefits of using indexes, but excessive use of indexes will cause abuse. Therefore, the index will also have its shortcomings: although the index greatly improves the query speed, it will also reduce the speed of updating the table, such as INSERT, UPDATE and DELETE on the table. Because when updating the table, MySQL not only needs to save the data, but also save the index file.
Creating index files will occupy disk space.
Normal index
Create index
This is the most basic index, it has no restrictions. It has the following creation methods:
CREATE INDEX indexName ON mytable(username(length));If it is CHAR, VARCHAR type, length can be less than the actual length of the field; if it is BLOB and TEXT type, length must be specified.
Modify the table structure
ALTER mytable ADD INDEX [indexName] ON (username(length))specifies directly when creating the table
CREATE TABLE mytable( ID INT NOT NULL, username VARCHAR(16) NOT NULL, INDEX [indexName] (username(length)) );'s syntax for deleting indexes
DROP INDEX [indexName] ON mytable;
Unique index
It is similar to the previous ordinary index, except that the value of the index column must be unique, but null values are allowed. In the case of a composite index, the combination of column values must be unique. It has the following creation methods:
Create index
CREATE UNIQUE INDEX indexName ON mytable(username(length))Modify table structure
ALTER mytable ADD UNIQUE [indexName] ON (username(length))Directly specify
CREATE TABLE mytable( ID INT NOT NULL, username VARCHAR(16) NOT NULL, UNIQUE [indexName] (username(length)) );
Use the ALTER command to add and delete indexes
There are four ways to add indexes to the data table:
##ALTER TABLE tbl_name ADD PRIMARY KEY (column_list ): This statement adds a primary key, which means that the index value must be unique and cannot be NULL.
ALTER TABLE tbl_name ADD UNIQUE index_name (column_list): The value of the index created by this statement must be unique (except NULL, NULL may appear multiple times ).
ALTER TABLE tbl_name ADD INDEX index_name (column_list): Add a normal index, the index value can appear multiple times.
ALTER TABLE tbl_name ADD FULLTEXT index_name (column_list):This statement specifies the index as FULLTEXT, which is used for full-text indexing.
mysql> ALTER TABLE testalter_tbl ADD INDEX (c);You can also use the DROP clause in the ALTER command to drop an index. Try the following example to delete the index:
mysql> ALTER TABLE testalter_tbl DROP INDEX (c)
mysql> ALTER TABLE testalter_tbl MODIFY i INT NOT NULL; mysql> ALTER TABLE testalter_tbl ADD PRIMARY KEY (i);You can also use the ALTER command to delete the primary key:
mysql> ALTER TABLE testalter_tbl DROP PRIMARY KEY;When deleting, you only need to specify PRIMARY KEY, but when deleting the index, You must know the index name.
mysql> SHOW INDEX FROM table_name\G ........
