MySQL Basics How to Insert How to Update How to Delete Database Management How to Add an Index How to Create a Table How to Delete a Table How to Rename a Table How to Truncate a Table How to Duplicate a Table
In this tutorial, we shall delete or drop a column from a table using MySQL DROP COLUMN statement. Syntax – Delete Column The syntax of MySQL DROP COLUMN is: ALTER TABLE table_name DROP COLUMN column_name; where table_name is the name of the table from which we are going to delete th...
delete from table where column in (values)语句执行慢的原因主要有两个: 删除操作是逐条执行的:当删除的数据量很大时,使用逐条删除的方式会导致删除操作非常耗时。 没有使用索引:如果没有为column列创建索引,查询引擎需要全表扫描来匹配需要删除的数据,这会导致删除操作变得非常慢。 3. 解决方案 为了解决上述问题,...
DELETE FROM tableName WHERE columnName = value; -- 删除表内的所有行: -- 即:保留表的结构、属性、索引 DELETE FROM tablename; DELETE * FROM tablename; 1. 2. 3. 4. 5. 6. 7. 删除同一张表内的所有内容 -- Truncate table语句用来删除/截断表里的所有数据 -- 和delete删除所有表数据在逻辑上...
案例增加mysql表一个字段ALTER TABLE 表名 ADD COLUMN 列名 varchar(50) DEFAULT null comment...
ALTER TABLE tbl_name ADD COLUMN col_name col_type, ALGORITHM=INPLACE, LOCK=NONE; ALGORITHM选项 INPLACE:替换:直接在原表上面执行DDL的操作。 COPY:复制:使用一种临时表的方式,克隆出一个临时表,在临时表上执行DDL,然后再把数据导入到临时表中,在重命名等。这期间需要多出一倍的磁盘空间来支撑这样的 操作...
系统表空间: 主要存储MySQL内部的数据字典数据,如information_schema下的数据。 用户表空间: 当开启innodb_file_per_table=1时,数据表从系统表空间独立出来存储在以table_name.ibd命令的数据文件中,结构信息存储在table_name.frm文件中。 Undo表空间: 存储Undo信息,如快照一致读和flashback都是利用undo信息。 从MySQL...
In MySQL >= 4.0 the number of rows deleted is returned; in MySQL 3.23 the number returned is always zero. Auto Increment Columns for MyISAM Tables If you have an auto increment primary key column in your MyISAM table the result will be slightly different depending which delete method you ...
I have DB, and in it, a table called "xf_user". Under that is a row called "sonnb_xengallery_video_count". There are lot of entries, over 2000, and I want to delete ALL of them from the DB using an SQL query. I can't seem to figure that out. Can anyone point me in the...
# ls-lh user1.ibd-rw-r---1mysql mysql 96K Nov612:48user.ibd 设置参数innodb_file_per_table=1时,创建表时会自动创建一个segment,同时分配一个extent,包含32个data page的来存储数据,这样创建的空表默认大小就是96KB,extent使用完之后会申请64个连接页,这样对于一些小表,或者undo segment,可以在开始时...