表的连接MySQLJoin：实现表的连接操作（mysqljoin实现）

MySQL is a popular relational database management system, using SQL Join to achieve table connection operations. MySQL supports several types of Join operations, such as inner join, left join, right join and full join. The specific introduction of each join operation is as follows:

1) Inner Join: Inner join is the most commonly used join operator. Two or more tables are connected by the same field in the same database. The inner join operator makes it easy to join values in different tables and retrieve query results. The SQL statement of inner join is as follows:

SELECT f1, f2… FROM table1 INNER JOIN table2 ON table1.common_field = table2.common_field

2) Left Join: Left join is used to find the values in the left table (by default) that are not in the right table. It returns all the rows of the left table and the corresponding rows in the right table. If the right table does not have a matching row in the left table, the row is still displayed with the value of the right table column is null. The left join SQL statement is as follows:

SELECT f1, f2… FROM table1 LEFT JOIN table2 ON table1.common_field = table2.common_field

3) Right Join: Right join is the opposite of left join and is used to find values in the right table (by default) that are not in the left table. It returns all the rows of the right table and the corresponding rows in the left table. If the left table does not have a matching row in the right table, the row is still displayed with the value of the left table column set to null. The right join SQL statement is as follows:

SELECT f1, f2… FROM table1 RIGHT JOIN table2 ON table1.common_field = table2.common_field

4) Full Join: Full join combines left join and right join. It returns all values in the left and right tables. If there is no value in one of the tables, null will be filled in the column of the table. The full join SQL statement is as follows:

SELECT f1, f2… FROM table1 FULL JOIN table2 ON table1.common_field = table2.common_field

In summary, MySQL Join is a very useful operator that allows us to connect the data in multiple tables and retrieve the information we need. We can choose a suitable join operator according to our actual needs, and using the correct SQL statement to achieve the corresponding table connection operation.