How Do SQL Joins Combine Data from Multiple Tables?

How to Join Two Tables in SQL

In SQL, joining tables allows you to combine data from multiple tables based on a common column. You can use different types of joins, such as inner join, left join, right join, and full outer join.

Here's an example of using a left outer join to fetch data from two tables, A and B:

Tables:

TABLE A	TABLE B
uid	name	uid	address
1	test1	1	address1
2	test2	2	address2
3	test3	4	address3
4	test4

Query:

SELECT A.uid, A.name, B.address
FROM A
LEFT JOIN B ON A.uid=B.uid;

Output:

uid	name	address
1	test1	address1
2	test2	address2
3	test3	NULL
4	test4	address3

As you can see, the result includes all rows from table A and the matching rows from table B. For the row with uid=3 in table A, there is no matching row in table B, so the address column is set to NULL.

To understand how joins work, it's helpful to visualize the process. For a left outer join, the rows from the left table (A in this case) are considered first. For each row in the left table, the join condition is evaluated to find matching rows in the right table (B in this case). If a match is found, the data from both rows is combined into a single result row. If no match is found, the data from the left table alone is included in the result row, with any missing columns from the right table set to NULL.

Refer to the image at https://www.codeproject.com/KB/database/Visual_SQL_Joins/Visual_SQL_JOINS_V2.png for a visual representation of joins.

The above is the detailed content of How Do SQL Joins Combine Data from Multiple Tables?. For more information, please follow other related articles on the PHP Chinese website!