alter table table_name add column test varchar(10) not null comment "测试" after empno --distinct --去除重复数据 select distinct column_name from emp; --多字段去除重复数据,只要一点不一样就算不一样 select distinct column1,column2 from emp; --加别名 --在查询过程中可以给列加别名,同时也可以...
Adding amulti-columnindex is very similar. You separate each column with a comma. And like single column indexes, you can also add a primary key as opposed to an index. [code type=”mysql”] CREATE TABLE Customer( LastName CHAR(30) NOT NULL, FirstName CHAR(30) NOT NULL, Email CHAR(...
# CONCAT(sl,s2,...) 函数返回结果为连接参数产生的字符串 若有任何一个参数为 NULL,则返回值为 NULL SELECT CONCAT('MySQL','5.7'),CONCAT('MySQL',NULL); # INSERT(s1,x,len,s2) 返回字符串 s1,子字符串起始于 x 位置,并且用 len 个字符长的字符串代替 s2 SELECT INSERT('Football',2,4,'Play...
ADD/DROP COLUMN operations aren't dependent on the table size anymore. In other words, the time taken to ADD/DROP new/old column(s) to/from a table with 1 row would be the same as time taken to ADD/DROP new/old column(s) to/from a table having 100M rows....
mysql> alter table t1 add column c3 char(10), ALGORITHM=INSTANT; Query OK, 0 rows affected (0.01 sec) Records: 0 Duplicates: 0 Warnings: 0 Although, we had few limitations in this early implementation. With ALGORITHM=INSTANT, new column can be added only as the last column of table. ...
issue 地址:https://bugs.mysql.com/bug.php?id=111538 影响范围: 从 8.0.29 版本开始, 在read heavy 场景, 性能可能有 5%~10% 的性能回退 MySQL 官方在8.0.29 里面加了instant add/drop column 能力, 能够实现 instant add 或者 drop cloumn 到表的任意位置. PolarDB 在这基础上增加了可以 Instant 修改...
field in Aspex_v_user_simple). I originally designed this in Access, before realising that there were some things that Access couldn't do, but when Access ran this query it would call the columns ThreadAuthor.user_id and PostAuthor.user_id. Is there any way to get MySQL to do the ...
to record the "default value" for each added column 4. The rows before instant add column would always have the old number of columns for its life time (until deleted). Such number of "core" rows will be stored in se_private_data of the mysql.tables 5. For "old" rows, the default...
MySQL从系统表读取表定义时,会将instant column相关的信息载入到InnoDB的表对象dict_table_t和索引对象dict_index_t中。 dict_table_t::n_instant_cols: 第一次instant add column之前的非虚拟字段个数(包含系统列), 具体过程详见函数dd_fill_dict_table ...
and the result is two columns with the same name, which was my problem also, especially if you use the MYSQL_ASSOC option for php's mysql_fetch_array. I dont understand why the result is not on the form "a.id" and "b.id", but the fix to this problem is this: ...