MySQL (2) query conditions, group query, outer join, subquery

1. Screening conditions

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​ 1.1, comparison operators

equal:=（注意！不是==）	greater or equal to:>=	null:is NULL
not equal to:！=或<>	less than:<	non empty: is NOT NULL
more than the:>	less than or equal to:<=

​ 1.2, logical operators

​ AND: andOR: orNOT:not

​ 1.3, sorting

​ Positive order (descending order):order by 字段

​ Conversely (descending order):order by 字段 desc

​ For example:select * from 表名 order by 字段

​ 1.4, Restrictions

​ - limit 数字: The limit condition can only output the first few lines

​ - limit num1, num2: Output num2 rows of data from the next position of num1

​ 1.5, deduplication

​ - distinct: Do not display duplicate data when searching

​ For example:select distinct * from 表名

​ 1.6, fuzzy query

​ - any number of characters:like '%'

​ For example:select * from 表名 where 字段 like '李%'

​ - any one character:like '_'

​ For example:select * from 表名 where 字段 like '李__'

​ 1.7, range query

​ - Numeric query:between num1 and num2

​ For example:select * from 表名 where 字段 between num1 and num2

​ -Interval return: in （条件，条件）, only filter the data that matches the brackets

​ For example:select * from 表名 where 字段 in（条件，条件...）

2. Aggregation and grouping (key and difficult points)

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​ 2.1, commonly used aggregation functions

Number of statistics: count (column)	Summing: sum(column)
**maximum value: **max(column)	**average:**avg(column)
**Minimum: **min(column)	**List all values ​​​​of the field: **group_concat(column)

​ Example: select count（*） as group_counts from 表名 group_by 字段: First group the table by field, calculate the number of each group, and finally store it in the group_counts field

​:select 字段 from 表名 group by 字段 List the contents of the field

​ 2.2, group query (group by)

2.3, aggregation screening

​ - having: Equivalent to where, you can filter the filtered table again

​:where和having的区别 where is to participate in the screening when viewing, and having is to be executed after the first screening, which is equivalent to re-screening the already-filtered tables.

​ - as: You can name the aggregate function and use it later

select * from (select * from students order by age) as e;
# (select * from students order by age) as e 这一步就是期别名为e

Three, subquery (understand)

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​ 3.1, leave the result of a query for the next query ( select 中嵌套 select)

​ Requirements:

​ 1): Nested inside the query

​ 2): Must always appear in parentheses (data in parentheses must be a single data when not aliased)

​ Example: select count(*) from grades where subject_number=(select number from subjects where title='Python1');: query subject_number is equal to the data in the following subquery (the data in the subquery must be a definite unique number)

Four, connection query

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​ 4.1, inner join (inner join)

​ -无条件连接：

​ Unconditional inner join, also known as cross join/Cartesian join

​ Each item in the first table will be combined with each item in the other table in turn

​ Example:MySQL> select * from 表名 join 表名

​ -有条件内连接：

​ On the basis of unconditional connection, add an on clause

​ When connecting, filter out those meaningful records for combination

​ Example: MySQL> select * from 表名 join 表名 on 表名.字段 = 表名.字段If you don’t want to display repeated data, change * to the desired data, and change the repeated to condition.field

​ 4.2, outer join (left/right join)

​ - 左外链接: (Based on the left table)

​ When two tables are linked, when the connection conditions do not match

​ Leave the data in the left table, while the data in the right table is filled with NULL

example:MySQL> select * from 表名 left join 表名 on 表名.字段 = 表名.字段

​ - 右外连接(Based on the table on the right)

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