MySQL - unique index

unique index

• The so-called unique index , When creating an index , The field value of the restricted index must be unique . Through this type of index, you can query a record faster than ordinary index .

  1. Define indexes when creating tables

  CREATE TABLE tablename(
      propname1 type1,
      ……
      propnamen type..n,
      UNIQUE INDEX | KEY [indexname] (propnamen [(length)] [ ASC | DESC ] ) );
  

  Be careful ：
  Parameters UNIQUE INDEX and UNIQUE KEY Is used to specify a field as an index , Choose one of the two ;
  Parameters indexname It's the index name , Omission ;
  Parameters propnamen Is the name of the field corresponding to the index , This field must be the one defined earlier and must be defined as UNIQUE constraint ;
  Parameters length Is an optional parameter , It refers to the length of the index , Must be a string type to use ;
  Parameters ASC and DESC All optional parameters ,ASC Indicates ascending order ,DESC Represents a descending order , If you don't specify , In ascending order .

  mysql> create table class(id int, name varchar(64), teacher varchar(64), UNIQUE INDEX name_index(name));	// establish class surface , At the same time specified name Is a unique index column 
  mysql> show create table class;						// Query table structure 
  mysql> insert into class values(1, ' Class one ', ' Zhang San ');	// insert data 1
  mysql> insert into class values(2, ' Class two ', ' Li Si ');	// insert data 2
  mysql> select * from class where id>0;				// Query by column 
  

  

  2. Create an index on an existing table
  Method 1 :

  CREATE UNIQUE INDEX indexname  
      ON tablename (propname [(length)] [ASC|DESC]);  
  

  

  Method 2 :

  ALTER TABLE tablename ADD UNIQUE INDEX | KEY indexname (propname [(length)] [ASC|DESC]);