[MySQL] classification of multi table queries

Multi table query classification

classification 1： Equivalent connection vs Non equivalent connection

1. Equivalent connection

SELECT employees.employee_id, employees.last_name, 
       employees.department_id, departments.department_id,
       departments.location_id
FROM   employees, departments
WHERE  employees.department_id = departments.department_id;

expand 1： Multiple connection conditions and AND The operator

expand 2： Distinguish between duplicate column names

• When there are the same columns in multiple tables , The column name must be prefixed with the table name .
• Columns with the same column name in different tables can be used Table name Distinguish .

SELECT employees.last_name, departments.department_name,employees.department_id
FROM employees, departments
WHERE employees.department_id = departments.department_id;

expand 3： The table alias

• Using aliases can simplify queries .

• Using table name prefix before column name can improve query efficiency .

SELECT e.employee_id, e.last_name, e.department_id,
       d.department_id, d.location_id
FROM   employees e , departments d
WHERE  e.department_id = d.department_id;

It should be noted that , If we use the alias of the table , In the query field 、 Only aliases can be used in the filter conditions , Cannot use the original table name , Otherwise you will report an error .

Ali Development Specification ：

【 mandatory 】 For queries and changes to table records in the database , As long as multiple tables are involved , You need to add the alias of the table before the column name （ or Table name ） To limit .

explain ： Query records for multiple tables 、 Update record 、 When deleting records , If there is no alias of the qualified table for the operation column （ Or watch name ）, And when the operation column exists in multiple tables , I'm going to throw an exception .

Example ：select t1.name from table_first as t1 , table_second as t2 where t1.id=t2.id;

Counter example ： In a business , Because the multi table associated query statement does not add the alias of the table （ Or watch name ） The limitation of , Two years after normal operation , Recently Add a field with the same name to a table , After making database changes in the pre release environment , The online query statement appears 1052 abnormal ：Column ‘name’ in field list is ambiguous.

expand 4： Join multiple tables

• summary ： Connect n Tables , Need at least n-1 Connection conditions . such as , Join three tables , At least two join conditions are required .

• practice ： Query the employee's last_name,department_name, city

SELECT e.last_name,d.department_name,l.city
FROM employees e,departments d,locations l
WHERE e.department_id = d.department_id
AND d.location_id = l.location_id

2. Non equivalent connection

SELECT e.last_name, e.salary, j.grade_level
FROM   employees e, job_grades j
WHERE  e.salary BETWEEN j.lowest_sal AND j.highest_sal;
 or 
WHERE e.salary >= j.lowest_sal AND e.salary <= j.highest_sal;

classification 2： Self join vs Non self connecting

• When table1 and table2 It's essentially the same table , Just use the alias to form two virtual tables to represent different meanings . Then the two tables are connected internally , External connection and other queries .

subject ： Inquire about employees surface , return “Xxx works for Xxx”

SELECT CONCAT(worker.last_name ,' works for ' 
       , manager.last_name)
FROM   employees worker, employees manager
WHERE  worker.manager_id = manager.employee_id ;

• Examples of self connection ：
  Query employees id, The name of the employee and the of his / her manager id And name

SELECT emp.employee_id,emp.last_name,mgr.employee_id,mgr.last_name
FROM employees emp ,employees mgr
WHERE emp.`manager_id` = mgr.`employee_id`;

classification 3： Internal connection vs External connection

• In addition to querying the records that meet the conditions , External join can also query records that one party does not meet the conditions .

• Internal connection ： Merging rows of more than two tables with the same column , There are no rows in the result set that do not match one table with another

SELECT employee_id,department_name
FROM employees e,departments d
WHERE e.`department_id` = d.department_id;  # Only 106 Bar record 

• External connection ： Merging rows of more than two tables with the same column , Except that the result set contains rows that match one table with another , The left table is also queried or Right table Mismatched rows .

• Classification of external connections ： The left outer join 、 Right connection 、 Full outer join

• The left outer join ： In addition to the rows that meet the connection conditions, the two tables also return the rows that do not meet the conditions in the left table , This connection is called The left outer join .

• Right connection ： In addition to the rows that meet the connection conditions, the two tables also return the rows that do not meet the conditions in the right table , This connection is called Right connection .

summary ：

• Internal connection : Merging rows of more than two tables with the same column , There are no rows in the result set that do not match one table with another .

• External connection : In addition to returning rows that meet the connection conditions during the connection of two tables Also return to the left （ Or right ） Rows in the table that do not meet the criteria , This connection is called left （ Or right ） External connection . When there are no matching rows , The corresponding column in the result table is empty (NULL).

• If it's a left outer connection , The table on the left of the join condition is also called Main table , The table on the right is called From the table .

• If it's a right outer connection , The table on the right in the join condition is also called Main table , The table on the left is called From the table .