Sự khác biệt giữa Join Where và Join And
Cho 2 bảng thế này:
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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;
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Với dữ liệu thế này:
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Bảng facebook:
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<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" />
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Bảng linkedin:
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<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" />
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Thì câu truy vấn join where này:
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SELECT *
FROM facebook
LEFT JOIN linkedin
ON facebook.name = linkedin.name
WHERE facebook.city = 'SF';
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Lại cho kết quả là:
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<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" />
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Trong khi sử dụng join and:
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SELECT *
FROM facebook
LEFT JOIN linkedin
ON facebook.name = linkedin.name AND facebook.city = 'SF';
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Lại cho kết quả là:
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<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" />
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Vì sao vậy?
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