tvd12
Enlightened
Asked At:
1670129447000
In:
SQL
Sự khác biệt giữa Join Where và Join And
Cho 2 bảng thế này:
<p>
</p>
<pre>
CREATE TABLE `facebook` (
`name` varchar(45) NOT NULL,
`city` varchar(45) DEFAULT NULL,
PRIMARY KEY (`name`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci;
CREATE TABLE `linkedin` (
`name` varchar(45) NOT NULL,
`city` varchar(45) DEFAULT NULL,
PRIMARY KEY (`name`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci;
</pre>
<p>
</p>
<p>
Với dữ liệu thế này:
</p>
<p>
</p>
<p>
Bảng facebook:
</p>
<img src="https://stackask.com/wp-content/uploads/2022/12/Screenshot-2022-12-04-at-18.47.29.png" alt="" class="alignnone size-full wp-image-8797" />
<p>
</p>
<p>
Bảng linkedin:
</p>
<img src="https://stackask.com/wp-content/uploads/2022/12/Screenshot-2022-12-04-at-18.48.23.png" alt="" class="alignnone size-full wp-image-8798" />
<p>
</p>
<p>
Thì câu truy vấn join where này:
</p>
<p>
</p>
<pre>
SELECT *
FROM facebook
LEFT JOIN linkedin
ON facebook.name = linkedin.name
WHERE facebook.city = 'SF';
</pre>
<p>
</p>
<p>
Lại cho kết quả là:
</p>
<p>
</p>
<img src="https://stackask.com/wp-content/uploads/2022/12/Screenshot-2022-12-04-at-18.49.32.png" alt="" class="alignnone size-full wp-image-8799" />
<p>
</p>
<p>
Trong khi sử dụng join and:
</p>
<p>
</p>
<pre>
SELECT *
FROM facebook
LEFT JOIN linkedin
ON facebook.name = linkedin.name AND facebook.city = 'SF';
</pre>
<p>
</p>
<p>
Lại cho kết quả là:
</p>
<p>
</p>
<img src="https://stackask.com/wp-content/uploads/2022/12/Screenshot-2022-12-04-at-18.50.31.png" alt="" class="alignnone size-full wp-image-8800" />
<p>
</p>
<p>
Vì sao vậy?
</p>
