Question

我想做的事情如下：

CREATE INDEX i_table_column 
ON table(column) 
IF NOT EXISTS INDEX ON table(column)

是否可以在MySQL中编写上述语句？

Answer 1

您可以更改Mitch Wheat提供的示例，以便检查给定列名称是否存在索引，但不检查索引名称本身：

delimiter //
DROP PROCEDURE IF EXISTS create_index_if_not_exists//
CREATE PROCEDURE `create_index_if_not_exists` (IN param_schema CHAR(255), IN param_table CHAR(255), IN param_column CHAR(255))
BEGIN

    SELECT @indexes := COUNT(*)
        FROM INFORMATION_SCHEMA.STATISTICS
        WHERE table_schema = param_schema
        AND table_name = param_table
        AND COLUMN_NAME = param_column;

    IF @indexes = 0 THEN
        SET @sql_cmd := CONCAT(
            'ALTER TABLE ',
            param_table,
            ' ADD INDEX ',
            '`', param_column, '` ',
            '(', param_column, ')');
        SELECT @sql_cmd;
        PREPARE stmt FROM @sql_cmd;
        EXECUTE stmt;
        DEALLOCATE PREPARE stmt;
    END IF;

END//
delimiter ;

示例：

DROP TABLE IF EXISTS `table1`;

CREATE TABLE `table1` (
  `id` int(10) unsigned NOT NULL auto_increment,
  `col1` int(10) unsigned,
  `col2` int(10) unsigned,
  PRIMARY KEY  (`id`)
) ENGINE=InnoDB;

CALL create_index_if_not_exists('test', 'table1', 'col2');

+--------------------------------------------+
| @sql_cmd                                   |
+--------------------------------------------+
| ALTER TABLE table1 ADD INDEX `col1` (col1) |
+--------------------------------------------+

SHOW CREATE TABLE `table1`;

CREATE TABLE `table1` (
  `id` int(10) unsigned NOT NULL auto_increment,
  `col1` int(10) unsigned default NULL,
  `col2` int(10) unsigned default NULL,
  PRIMARY KEY  (`id`),
  KEY `col1` (`col1`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1

以下行显示要执行的命令，如有必要，可以从存储过程中删除：

SELECT @sql_cmd;

如何用MySQL有条件地创建索引？

1 个答案: