[MySQL] multi table query

Multi-table query

• Multi-table query , Also known as associative queries , It means that two or more tables complete the query operation together .

• Prerequisite ： There is a relationship between these tables queried together （ one-on-one 、 One to many ）, There must be associated fields between them , This associated field may establish a foreign key , Or you may not have established a foreign key . such as ： Employee table and department table , These two tables depend on “ Department number ” Association .

1. Multi table connection caused by a case

1.1 Case description

• Check the name of the employee ’Abel’ Which city do people work in ？

SELECT * 
FROM employees
WHERE last_name = 'Abel';

SELECT *
FROM departments
WHERE department_id = 80;


SELECT *
FROM locations 
WHERE location_id = 2500;

• Getting data from multiple tables ：
  

• Case study ： Query the employee's name and department name

SELECT last_name, department_name
FROM employees, departments;

• Query results ：

+-----------+----------------------+
| last_name | department_name      |
+-----------+----------------------+
| King      | Administration       |
| King      | Marketing            |
| King      | Purchasing           |
| King      | Human Resources      |
| King      | Shipping             |
| King      | IT                   |
| King      | Public Relations     |
| King      | Sales                |
| King      | Executive            |
| King      | Finance              |
| King      | Accounting           |
| King      | Treasury             |
...
| Gietz     | IT Support           |
| Gietz     | NOC                  |
| Gietz     | IT Helpdesk          |
| Gietz     | Government Sales     |
| Gietz     | Retail Sales         |
| Gietz     | Recruiting           |
| Gietz     | Payroll              |
+-----------+----------------------+
2889 rows in set (0.01 sec)

• Analyze the error ：

SELECT COUNT(employee_id) FROM employees;
# Output 107 That's ok 

SELECT COUNT(department_id)FROM departments;
# Output 27 That's ok 

SELECT 107*27 FROM dual;

We call the problem in the above multi table query as ： The error of Cartesian product .

• There is a Cartesian product error , Reason for the error ： The connection condition of multiple tables is missing

# Wrong implementation ： Each employee matched with each department .
SELECT employee_id,department_name
FROM employees,departments;  # Query out 2889 Bar record 

# Wrong way 
SELECT employee_id,department_name
FROM employees CROSS JOIN departments;# Query out 2889 Bar record 

1.2 The cartesian product （ Or cross connect ） The understanding of the

• Cartesian product is a mathematical operation . Suppose there are two sets X and Y, that X and Y The Cartesian product of is X and Y All possible combinations of , That is, the first object comes from X, The second object comes from Y All the possibilities of . The number of combinations is the product of the number of elements in the two sets .

• SQL92 in , Cartesian product is also called Cross connect , English is CROSS JOIN. stay SQL99 It is also used in CROSS JOIN Means cross connect . Its function is to connect any table , Even if the two tables are not related . stay MySQL Cartesian product will appear in the following cases ：

• Query employee name and department name

SELECT last_name,department_name FROM employees,departments;
SELECT last_name,department_name FROM employees CROSS JOIN departments;
SELECT last_name,department_name FROM employees INNER JOIN departments;
SELECT last_name,department_name FROM employees JOIN departments;

1.3 Case analysis and problem solving

• The error of Cartesian product will occur under the following conditions ：

  • Omit join conditions for multiple tables （ Or related conditions ）
  • Connection condition （ Or related conditions ） Invalid
  • All rows in all tables are interconnected

• To avoid Cartesian product , Sure stay WHERE Add a valid connection condition .

• After adding connection conditions , The query syntax ：

  SELECT	table1.column, table2.column
  FROM	table1, table2
  WHERE	table1.column1 = table2.column2;  # Connection condition 

• stay WHERE Clause .

• Write it correctly ：

Case study ： Query the employee's name and department name

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

• When there are the same columns in the table , Prefix the column name with the table name .

• If there are fields in multiple tables in the query statement , You must specify the table where this field is located .

SELECT employees.employee_id,departments.department_name,employees.department_id
FROM employees,departments
WHERE employees.`department_id = departments.department_id;

• Suggest ： from sql Optimization angle , It is recommended to query multiple tables , Each field is preceded by the table in which it is located .

• You can start the watch Alias , stay SELECT and WHERE Aliases for tables used in .

SELECT emp.employee_id,dept.department_name,emp.department_id
FROM employees emp,departments dept
WHERE emp.department_id = dept.department_id;

• If you alias a table , Once in a SELECT or WHERE If table names are used in , You must use the alias of the table , Instead of using the original name of the table .

# The following operation is wrong ：
SELECT emp.employee_id,departments.department_name,emp.department_id
FROM employees emp,departments dept
WHERE emp.department_id = departments.department_id;

• Conclusion ： If there is n Multiple tables realize the query of multiple tables , At least n-1 Connection conditions
• practice ： Check the employee's employee_id,last_name,department_name,city

SELECT e.employee_id,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;