DBA--创建索引

CREATE TABLE t_user1(id INT ,                     userName VARCHAR(20),                     PASSWORD VARCHAR(20),                     INDEX (userName)             );             CREATE TABLE t_user2(id INT ,                     userName VARCHAR(20),                     PASSWORD VARCHAR(20),                     UNIQUE INDEX index_userName(userName)             );           CREATE TABLE t_user3(id INT ,                     userName VARCHAR(20),                     PASSWORD VARCHAR(20),                     INDEX index_userName_password(userName,PASSWORD)             );             CREATE INDEX index_userName ON t_user4(userName);CREATE UNIQUE INDEX index_userName ON t_user4(userName);CREATE  INDEX index_userName_password ON t_user4(userName,PASSWORD);ALTER TABLE t_user5 ADD INDEX index_userName(userName);ALTER TABLE t_user5 ADD UNIQUE INDEX index_userName(userName);ALTER TABLE t_user5 ADD INDEX index_userName_password(userName,PASSWORD);DROP INDEX index_userName ON t_user5;DROP INDEX index_userName_password ON t_user5;