MySql learning 4: multi-table query

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multi-table relationship

There are various association relationships between each table structure, which are basically divided into three types: one-to-many (many-to-one), many-to-many, and one-to-one

One-to-many (many-to-one)
example: the relationship between departments and employees. One department corresponds to multiple employees, and one employee corresponds to one department (regardless of the fact that one employee corresponds to multiple departments)

Realization: Create a foreign key on the many side, pointing to the primary key on the one side

Many-to-many
Example: Student-Course relationship. A student can choose multiple courses, and a course can also be chosen by multiple students

Realization: Create a third intermediate table. The intermediate table contains at least two foreign keys, which are associated with two primary keys


one-to-one
. For example: the relationship between users and user details. The one-to-one relationship is mostly used for single-table splitting. The basic fields of one table are placed in one table, and other detailed fields are placed in another table to improve operational efficiency.

For example: to improve query performance, when there is too much data in a table, the query will become slow; it can make the database structure clearer and the structure of each table simpler

You can add a foreign key on either side, associate the primary key on the other side, and set the foreign key to be unique.

Overview of multi-table queries

Refers to querying data from multiple tables.

Cartesian Product

Cartesian product: refers to all combinations of two sets A and B in mathematics. Need to eliminate invalid Cartesian

integral

• connection query

  • Inner join: equivalent to querying the intersection of A and B
  • Outer join:
    • Left outer join: Query all data in the left table, and some data in the intersection of two tables
    • Right outer join: Query all data in the right table, and some data in the intersection of two tables
  • Self-join: the connection query between the current table and itself, the self-join must use the table alias

• subquery

inner join

The inner join query is the intersection of two tables

implicit inner join

select 字段 from 表1,表2 where 条件

Show Inner Links

select 字段列表 from 表1 join 表2 on 连接条件

Query the employee's job number, name and department name

// 隐式内连接
select emp.workno,emp.name,dept.dept_name  from emp,dept where emp.dept_id = dept.id;
// 显示外连接
select emp.workno,emp.name,dept.dept_name from emp join dept on emp.dept_id = dept.id;

outer join

left outer join

select 字段 from 表1 left join 表2 on 条件

All the data in the query emptable and the corresponding department information

select emp.*,dept.dept_name  from emp left join dept on emp.dept_id = dept.id;

The difference between this and the inner join is that if there is a new employee who has just joined the job and does not have a corresponding department, the inner join will not query the employee's information, but the left outer join will query the information.

Inner joins must be present on both sides; left outer joins must be present on the left side, but not on the right side

right outer join

select 字段 from 表1 right join 表2 on 条件

All the data queried dept, and the information of the corresponding employee table

self-join

select 字段列表 from 表A join 表A 别名B  on 条件

A self-join query can be an inner join query or an outer join query.
For example, when the data in a table has a hierarchical structure, you can use a self-join to query parent-child or hierarchical relationships. For example, query the employee's superior or subordinate in the employee table.

Self-join can help us compare and analyze operations in the same table, and solve some complex query requirements. However, it should be noted that when using self-connection, the connection conditions need to be carefully handled to avoid infinite loops or performance problems.

select emp.*,a.name  from emp join emp as a where emp.manager_id = a.id ;

joint query

Joint query is to combine the results of multiple queries to form a new query result set. For example, query the employees whose salary is less than 5000 and the employees whose age is greater than 60.

Keyword: union all and union, the latter is a deduplication of the former.

During joint query, the number of columns and field types of multiple tables must be consistent.

select 字段列表 from a 条件
union 
select 字段列表 from b 条件

subquery

Nested statements in SQL selectare called nested queries, also known as subqueries.

select * from t1 where column1 = (select column1 from t2)

The outer statement of the subquery can be any of insert, delete, update,select

According to the result of the subquery, it can be divided into:

• scalar subquery, the subquery result is a single value
• Column subquery, the result of the subquery is a column
• row subquery, the result of the subquery is one row
• Table subquery, the result of the subquery is multiple rows and multiple columns

scalar subqueries

A subquery that returns a single value (number, string, date, etc.) is called a scalar subquery.

Commonly used operators are: >, <, !=,=

Query all employee information of the R&D center:
1) First query the department id of the R&D center
2) Query employee information according to the department id

select * from emp where dept_id = (select id from dept where dept_name = '研发中心')

column query

The result returned by a subquery is one column (can be multiple columns). This subquery is called a column subquery. The
commonly used operators are: in, not in, and, some,all

Query all employee information of the delivery center and R&D center
1) Query the department id of the delivery center and R&D center
2) Query employee information according to the department id

select * from emp where dept_id  in (select id from dept where dept_name in ('交付中心','研发中心'))

row subquery

The result returned by a subquery is one row (can be multiple columns). This subquery is called a row subquery. The
commonly used operators are: >, <, !=,=

Query employee information of the same department as Qin Yi's gender
1) Query Qin Yi's gender and department id
2) Query employee information based on gender and department id

select * from emp where (sex ,dept_id) = (select sex ,dept_id  from emp where name = '秦一')

table subquery

The result returned by a subquery is multiple rows and multiple columns. This subquery is called a table subquery.

Commonly used operators:in

Query employee information with the same gender and department as Qin Yi and Li Er

select * from emp where (sex ,dept_id) in (select sex ,dept_id  from emp where name = '秦一' or name = '李二')

Query the employee information after the entry date 2023-06-12, and its department information

select e.*,d.* from (select * from emp where entrydata > '2023-06-12') as e 
left join dept as d on e.dept_id = d.id