INNER JOIN only retains two table matching rows, LEFT JOIN retains all data in the left table, RIGHT JOIN retains all data in the right table, FULL OUTER JOIN retains all records, and CROSS JOIN generates Cartesian product. INNER JOIN is suitable for situations where both table data exist; LEFT JOIN is used to list all records in the left table and supplement the right table information; RIGHT JOIN is used similar to LEFT JOIN but focuses on the right table; FULL OUTER JOIN is used to compare and analyze data that are inconsistent on both sides; CROSS JOIN is used to generate all combinations but should be used with caution. Each JOIN has a clear scenario, and correct use can improve query efficiency and accuracy.

SQL JOIN is one of the most core operations in database queries, which determines how you extract data from multiple tables. Understanding different types of JOINs and their usage scenarios will allow you to write more efficient and accurate query statements.

INNER JOIN: Only matching data is retained

INNER JOIN is one of the most commonly used JOIN types. Its purpose is to return only matching rows in two tables , and the mismatched part will not appear in the result.

For example, if you have two tables, one is user table and the other is order table, you can use INNER JOIN:

 SELECT users.name, orders.order_id
FROM users
INNER JOIN orders ON users.user_id = orders.user_id;

This statement will return all results that have corresponding records in users and orders tables. If a user has no order, or if an order has no corresponding user (such as an exception in data), it will not appear in the result.

Applicable scenarios:

• You need to make sure that the data in both tables exist before presenting it.
• Reports or statistical analysis with high data integrity requirements.

LEFT JOIN: Keep all data in the left table

The feature of LEFT JOIN is that all records in the left table (main table) will be retained , and even if the right table has no match, the right field will be displayed as NULL.

Continue to the example above, if you want to list all users, regardless of whether there is an order or not, you can write it like this:

 SELECT users.name, orders.order_id
FROM users
LEFT JOIN orders ON users.user_id = orders.user_id;

This way you can see the order information of all users, even if some users have not placed an order yet.

Common uses include:

• Statistics whether each user is active (for example, whether there is an order or whether there is a login record)
• Check the situation of missing associated data (such as having users but no orders)

Note:

• If there are multiple matching records on the right table, the results will be repeated. Need to be used with GROUP BY or DISTINCT.
• The query result may contain NULL values, be careful when processing.

RIGHT JOIN vs. FULL OUTER JOIN: Less used but useful

RIGHT JOIN is a mirror of LEFT JOIN, which retains all records in the right table , and NULL is added when the left table does not match. In practice, the frequency of use is not high, because the same effect can be achieved by adding LEFT JOIN to the position of the exchange table.

FULL OUTER JOIN is a combination of the two, and all records are returned regardless of whether the left and right tables match . It is very useful for some complex comparison analyses, such as finding data that are inconsistent on both sides.

For example, if you want to find out which users do not have orders and which orders do not have users, you can use:

 SELECT users.name, orders.order_id
FROM users
FULL OUTER JOIN orders ON users.user_id = orders.user_id
WHERE users.user_id IS NULL OR orders.user_id IS NULL;

This type of JOIN is very useful in some data analysis scenarios, but it does not support FULL OUTER JOIN in MySQL and requires UNION simulation.

CROSS JOIN: Cartesian production, use it with caution!

CROSS JOIN is a special type that combines every row on the left table and every row on the right table to produce a Cartesian product. That is to say, if there are 10 records in Table A and 20 records in Table B, there will be 200 records in the result.

This type of JOIN is rarely used for daily queries, but may be used in the following situations:

• Generate all possible combinations (such as product specifications)
• Build time dimension or other dimension cross data

Be careful, though, as it can easily lead to explosive growth in the result set.

Basically that's it. Although there are not many types of JOIN, each has a clear usage scenario. If you use it right, you will get twice the result with half the effort. If you use it wrong, you may not find the desired data, and may even slow down the system performance.

The above is the detailed content of Understanding the various SQL JOIN types and their applications.. For more information, please follow other related articles on the PHP Chinese website!