Detailed explanation of the example of SQL connection operation left connection query (LEFT JOIN) in MySQL

Preface

Left Join Query (LEFT JOIN) in MySQL is a SQL join operation that combines rows in two or more tables based on matching relationships between the two tables. A left join returns all rows of the left table (the table specified in the FROM clause), even if there are no matching rows in the right table (the table specified in the JOIN clause). If there are no matching rows in the right table, the result set is populated with NULL on the column corresponding to the right table.

Basic syntax

The basic syntax for left connection is as follows:

SELECT columns
FROM table1
LEFT JOIN table2
ON table1.column_name = table2.column_name;

SELECT columns: Specify the column to query.
FROM table1: Specify the main table (left table).
LEFT JOIN table2: Specify the table to be joined to the left table (right table).
ON table1.column_name = table2.column_name: Specify the join conditions, that is, how to match between two tables.

Example

Suppose there are two tables:employees(employee form) anddepartments(Department Table), they passdepartment_idRelated.

-- employees surface
+----+----------+----------+
| id | name     | department_id |
+----+----------+----------+
| 1  | John     | 1        |
| 2  | Doe      | 2        |
| 3  | Smith    | 3        |
+----+----------+----------+

-- departments surface
+----+------------+
| id | department |
+----+------------+
| 1  | Sales     |
| 2  | Marketing |
+----+------------+

Now we want to query the names of all employees and the names of the department they belong to, even if some employees are not assigned to the department.

SELECT , 
FROM employees
LEFT JOIN departments ON employees.department_id = ;

result

+----------+------------+
| name     | department |
+----------+------------+
| John     | Sales      |
| Doe      | Marketing  |
| Smith    | NULL       |
+----------+------------+

In this result, all employees are listed, evenSmithNot assigned to any department, hisdepartmentThe column is displayed asNULL。

Things to note

performance: In some cases, left-connection can cause performance issues, especially when handling large amounts of data. Optimizing queries and indexes can improve performance.
NULL value processing: When using left connection, you need to pay attention to handling the NULL values in the result set to avoid problems in the application logic.
Multi-table connection: Left connections can be used in conjunction with other types of connections such as internal connection INNER JOIN to achieve more complex query requirements.

Left join is a powerful tool for handling table-associated queries in relational databases, allowing developers to flexibly access and combine data from different tables.

Summarize

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