JAVA single row diary -2020 / 2/8-MySQL-DML

1. Insert (add) data

1. Assignment to a column

insert into 表名(列1,列2) values (值1,值2);

2. To assign all the columns

insert into 表名 values (值1,值2,..,值n)

In addition to intthe type of data NULL, other types of data all with ''or ""caused by

2. The data look-up table

• check sentence

select 
	字段列表
from
	表名列表
where
	条件列表
group by
	分组字段
having
	分组之后的条件
order by
	排序
limit
	分页限定

• Underlying query

1. Query multiple fields

select 字段名1,字段名2... from 表名;

select * from 表名;  #查询所有字段

2. remove duplicates

select distinct 字段n from 表名;  #去除表中字段n中的重复元素

3. Calculated column
     ● Calcd can use a series of four operations;
     ● nullparticipation calculation, the calculation result are null. It may be used ifnull(表达式1，表达式2), if the expression 1 is null, replacing an expression put expression 1 2
4. Surnamed

select 字段名1 as 别名 from 表名;

select 字段名1 别名 from 表名;

• Conditions inquiry

1. where clause followed by the condition
2. Operators

Operators	Features
>、<、<=、>=、=、<>	Greater than, less than, greater than or equal, less, equal, non-
and 或 &&	versus
or 或 ||	or
or not!	non-
between A and B	Between A and B (including A and B)
in (collection)	Query data in the collection of the
is null	Null query data (can not use = null)
like	Fuzzy queries. Placeholders: any single character _any number of characters%

3. Delete the data in the table

1. To delete an item in the table

delete from 表名 where 条件;

2. Delete all the data in the table

delete from 表名;            逐条删除,执行效率低,不推荐使用

truncate table 表名;         删除表,再新建一个一样的,推荐使用

4. Modify the data in the table

update 表名 set 列1=值1,列2=值2,列3=值3,...,列n=值n,where 条件; #不加where条件则表中所有数据都会改变