Difference between left and right connections in the database _ MySQL

Differences between left and right connections in databases Differences between left and right connections in databases

Today, someone asked me a question: What is the difference between left and right connections in the database? If there are two tables A and B, there are three data records in Table A and four data records in Table B. through the left and right connections, what is the minimum number of data records queried? What is the maximum number?

I was asked this question, and then I asked the database developer, the results are various:

A max 12 min 0

Maximum 12 minimum unknown B

C. The maximum unknown minimum is 3.

D. maximum 12. minimum 3

E unclear

1. description

(1) left join: data can be retrieved as long as there are records in the left table,The right side has

Only records in the left table can be retrieved.

(2) Right join: the right join is to retrieve data as long as there are records in the right table.

2. Examples

1. Create two tables: t_left_tab and t_right_tab.
2. Use t_left_tab as the left table and t_right_tab as the right table.

Left join: SELECT * FROM t_left_tab a left join t_right_tab B ON a. 'id' = B. 'id ';

Query Results:

Right JOIN: SELECT * FROM t_right_tab a left join t_left_tab B ON a. 'id' = B. 'id ';

Query Results:

Maximum number of queries: SELECT * FROM t_left_tab a left join t_right_tab B ON 1 = 1;

Query Results:

3. Summary

The difference between left and right connections of database A: The master table is different.

B connects to the right through the left connection. the minimum number of records is 3 (the number of records is smaller), and the maximum number is 12 (3 × 4)