SQL外键引用

问题描述

$ b $ pre $ FOREIGN KEY（a）参考A（a），FOREIGN KEY（b）REFERENCES A（b）


与

  FOREIGN KEY（a，b）参考A（a，b）？ 
  

解决方案

b $ b

有两个引用语句意味着 a 和 b 独立出现在 A 中。也就是说，它们是有效的价值，但不一定要一起出现。想想二月三十日。有一个有效的月份和一个月的有效日期，当每个单独检查。

拥有一个引用语句表示 a 和 b 出现在 A 一起。因此，2月30日将不能通过这个测试。传统上，外键指的是主键或唯一键。这不是在MySQL中强制执行的，但它是一个有用的指南（在实践中也是有用的）。因此，在两个引用的例子中，这通常意味着 a 和 b 是唯一的。虽然这不是强制执行的，但它可以很好地说明如何使用这些密钥。在第二种情况下， a 和 b 的组合是唯一的。

Does

FOREIGN KEY (a) REFERENCES A(a),FOREIGN KEY (b) REFERENCES A(b)

Has the same meanning as in :

FOREIGN KEY (a,b) REFERENCES A(a,b)?

解决方案

No.

Having two references statement means that both a and b appear in A independently. That is, they are valid values, but they don't have to appear together. Think of "February 30". It has a valid month and a valid day of month, when each are checked separately.

Having a single references statement means that a and b appear in A together. Hence, "February 30" would fail this test.

Traditionally, a foreign keys refer to a primary or unique key. This is not enforced in MySQL, but it is a useful guideline (and also useful in practice). So, in the example with two references, this would usually mean that both a and b are unique. Although this is not enforced, it gives a good indication of how the keys will be used. In the second case, the combination of a and b is unique.

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