SQL multi table query

Multi-table query

basic sql sentence

SELECT < Target field > FROM < Database table > WHERE < Query criteria > GROUP BY < Group by > ORDER BY < Arrangement basis >

Multi-table query

1. Internal connection ： Return the data that satisfies the connection condition

Format ：SELECT < Target field > FROM <INNER JOIN+ Table name +ON+ Connection condition > WHERE < Query criteria >

Be careful ： Clarify the relationship between tables , Find out what data from which tables , What are the connection conditions between tables

Patients with a ： Double table query

Example 2 ： Multi-table query

a  sys_user       user_name => user_id dept_id
b  sys_user_role  user_id   => role_id
c  sys_role       role_id   => role_name
d  sys_role_dept  role_id   => dept_id
e  sys_dept       dept_id   => dept_name parent_id dept_name
f  sys_dept       parent_id => dept_name

SELECT 
	a.user_name,
	a.user_id,
	a.dept_id,
	b.role_id,
	c.role_name,
	e.dept_name,
	f.dept_name
FROM
	sys_user AS a
	INNER JOIN  sys_user_role AS b  ON b.user_id = a.user_id
    INNER JOIN  sys_role AS c       ON c.role_id = b.role_id
    INNER JOIN  sys_dept AS e       ON e.dept_id = a.dept_id
	INNER JOIN  sys_dept AS f       ON f.dept_id = e.parent_id
WHERE
	a.user_name = 'zjzsqxyf01'
    

2. External connection ： The external connection lists not only the rows that match the connection conditions , Instead, list the left table （ Left outer connection ）、 Right table （ Right outer connection ） Or two tables （ When fully externally connected ） All data rows that meet the search criteria in the

   （1） Left connection ： Return all data in the left table

   Format ：SELECT < Target field > FROM <LEFT JOIN+ Table name +ON+ Connection condition > WHERE < Query criteria >

   

   （2） The right connection ： Return all data in the right table

   Format ：SELECT < Target field > FROM <RIGHT JOIN+ Table name +ON+ Connection condition > WHERE < Query criteria >

   

   （3） Full connection ： On the basis of equivalent connection, add the unmatched data of the left table and the right table

   Format ：SELECT < Target field > FROM <FULL JOIN+ Table name +ON+ Connection condition > WHERE < Query criteria >

3. Natural join ： Based on all columns with the same fields in two tables , Return the data that meets the query conditions in the two tables