MySQL中常用的JOIN類型包括INNER JOIN、LEFT JOIN、RIGHT JOIN、FULL OUTER JOIN（需模擬）和CROSS JOIN。 INNER JOIN僅返回兩表中匹配的行；LEFT JOIN返回左表所有行，無(wú)匹配時(shí)右表字段為NULL；RIGHT JOIN返回右表所有行，無(wú)匹配時(shí)左表字段為NULL；FULL OUTER JOIN需通過LEFT JOIN與RIGHT JOIN加UNION實(shí)現(xiàn)，返回兩表所有行；CROSS JOIN生成兩表各行的所有組合。選擇合適的JOIN類型可精準(zhǔn)獲取所需數(shù)據(jù)。

When you're working with multiple tables in MySQL, JOIN operations are essential for combining related data. Understanding the different types of JOINs helps you pull exactly the data you need efficiently. Here's a breakdown of the most commonly used JOIN types and when to use them.

INNER JOIN: Get Matching Rows Only

The INNER JOIN is the most common type. It returns rows only when there's a match in both tables based on the join condition.

For example, if you have a users table and an orders table, using INNER JOIN will give you only those users who have placed orders — not users without orders or orders without matching user records.

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

This is useful when you're sure that you only want data where relationships exist between tables. If either side might be missing data, this isn't the one to use.

LEFT JOIN (or LEFT OUTER JOIN): Include All Left Table Rows

A LEFT JOIN includes all the rows from the left table (the one mentioned first), even if there's no match in the right table. If there's no match, the result will still include the row but with NULL values for columns from the right table.

Say you want a list of all users, along with their orders if they've made any:

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

This gives you every user, whether or not they've placed an order. This can be especially handy when auditing data or generating reports that require full lists with optional details.

If you want to find users without orders, just add:

• WHERE orders.order_id IS NULL

That filters out users who do have order records.

RIGHT JOIN (or RIGHT OUTER JOIN): Opposite of LEFT JOIN

The RIGHT JOIN works like the LEFT JOIN, but it keeps all rows from the right table instead. While less commonly used, it can be helpful in specific cases — for example, when you want to list all orders, including those that may not yet be tied to a known user.

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

This would show all orders, even if the associated user has been deleted or doesn't exist yet. In practice, many developers prefer using LEFT JOINs and switching the table order rather than using RIGHT JOINs, just because it feels more intuitive.

FULL OUTER JOIN: Combine All Rows from Both Tables

MySQL does not support FULL OUTER JOIN directly, but you can simulate it using a combination of LEFT JOIN and RIGHT JOIN with UNION .

This type of join gives you all records from both tables, matching where possible and filling in NULLs where there's no match.

Here's how you'd write it in MySQL:

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

Use this when you absolutely need to see all data from both sides, regardless of matches. Be cautious though — it can return a lot of data and sometimes a lot of NULLs.

CROSS JOIN: Pair Everything with Everything

A CROSS JOIN pairs every row in the first table with every row in the second table. That means if table A has 10 rows and table B has 20 rows, you'll get 200 results.

It's not often needed, but comes in handy for generating combinations — like pairing each product with each color option.

 SELECT products.name, colors.color
FROM products
CROSS JOIN colors;

This kind of JOIN should be used carefully, as it can quickly create massive datasets.

Depending on your goal, choosing the right JOIN type makes a big difference in what data you end up with. INNER JOIN keeps things tight, LEFT JOIN expands the set from one side, and others like CROSS or simulated FULL JOINs handle special cases.

You don't always need to remember every syntax by heart — just understand the logic behind each type, and you'll know which one to reach for next time.

基本上就這些。

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