MySQL 是否在表间连接期间自动使用合并函数？

Question

在 table 加入期间，MySQL 何时使用此功能？

The single result column that replaces two common columns is defined using the coalesce operation. That is, for two t1.a and t2.a the resulting single join column a is defined as a = COALESCE(t1.a, t2.a), where:
COALESCE(x, y) = (CASE WHEN x IS NOT NULL THEN x ELSE y END)

https://dev.mysql.com/doc/refman/8.0/en/join.html

我知道这个函数是干什么的，但是我想知道在join操作的时候是什么时候用到的。这对我来说毫无意义！有人可以给我举个例子吗？

Answer 1

参考自然连接和使用连接时的冗余列消除。描述列如何从显示中排除。

操作顺序在您引用的部分上方进行了描述。

First, coalesced common columns of the two joined tables, in the order in which they occur in the first table

Second, columns unique to the first table, in order in which they occur in that table

Third, columns unique to the second table, in order in which they occur in that table

例子

t1

| a | b | c |
| 1 | 1 | 1 |

t2

| a | b | d |
| 1 | 1 | 1 |

与using的连接

SELECT * FROM t1 JOIN t2 USING (b);

将导致 t1.b 合并（由于 USING），然后是第一个 table 独有的列，然后是第二个 table 中的列].

| b | a | c | a | d |
| 1 | 1 | 1 | 1 | 1 |

而自然连接

SELECT * FROM t1 NATURAL JOIN t2;

将导致合并 t1 列（或者更确切地说，来自两个 table 的公共列），然后是第一个 table 的唯一列，然后是那些在第二个 table.

| a | b | c | d |
| 1 | 1 | 1 | 1 |

MySQL 是否在表间连接期间自动使用合并函数？

Does MySQL automatically use the coalesce function during a join between tables?

mysql

join

coalesce