INNER JOIN僅返回兩表中匹配的行，而LEFT JOIN返回左表所有行，即使右表無(wú)匹配。例如，使用INNER JOIN連接users和orders表時(shí)，僅包含有訂單的用戶；而LEFT JOIN則包含所有用戶，未下單用戶的訂單字段為NULL。選擇JOIN類型時(shí)需注意：需查找未匹配記錄用LEFT JOIN并篩選NULL值；避免重復(fù)數(shù)據(jù)選INNER JOIN；使用LEFT JOIN時(shí)注意聚合函數(shù)可能導(dǎo)致的數(shù)據(jù)膨脹；始終檢查ON條件以確保正確關(guān)聯(lián)。理解兩者處理非匹配行的方式是正確使用的關(guān)鍵。

When you're working with MySQL and need to combine data from multiple tables, JOINs are essential. The difference between INNER JOIN and LEFT JOIN is pretty straightforward once you understand how each one handles matching and non-matching rows.

Here’s what you need to know:

• INNER JOIN only returns rows where there's a match in both tables.
• LEFT JOIN (or LEFT OUTER JOIN) returns all rows from the left table, even if there's no match in the right table.

Let’s break this down further so you can see when and why you’d use each one.

When to Use INNER JOIN

Use an INNER JOIN when you only care about records that have corresponding entries in both tables. If there's no match in the second table, those rows won’t show up at all in your result set.

For example, imagine you have two tables:

• users (id, name)
• orders (user_id, total)

If you run:

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

You’ll get a list of names and their order totals — but only for users who actually have placed orders. Users without any orders? They’re not included.

This makes sense if you're generating reports or analyzing relationships that must exist on both sides.

When to Use LEFT JOIN

A LEFT JOIN is useful when you want all the records from the first (left) table, regardless of whether they have matches in the second (right) table.

Using the same users and orders tables:

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

Now, every user shows up — even if they haven't placed an order yet. For those without orders, the total field will be NULL.

This kind of query is helpful when you're auditing data, checking for missing relationships, or building dashboards where visibility into all entities is important.

Practical Tips for Choosing Between Them

Here are a few quick tips to help you decide which JOIN type to use:

• Need to find unmatched records? Try LEFT JOIN and filter where the right-side columns are NULL.
• Want to avoid duplicates or partial data? INNER JOIN ensures clean pairings.
• Be careful with aggregate functions like COUNT or SUM after a LEFT JOIN — it might inflate numbers if one row matches multiple rows on the other side.
• Always double-check your ON condition — a small mistake can lead to unexpected results or performance issues.

So yeah, the main thing is understanding how each JOIN treats unmatched rows. INNER JOIN filters them out, LEFT JOIN keeps them. Once you've used them a few times in real scenarios, it becomes second nature.

基本上就這些。

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