Question

概念疑问：

我尝试过很多东西（MySQL）但是这个更新工作的唯一方法就是我在这里描述的方式。它有效，但我想确定一下：

当我们使用加入更新时，我设置的值来自正在加入的表，有多个加入每行的可能性，更新是否始终是第一个可能的行来进行连接？

create table letter (id_letter bigint, id_group_table bigint, letter char(1));  
create table group_table (id_group_table bigint, id_whatever bigint, champion char(1));
create table whatever (id_whatever bigint);

insert into whatever values (1);
insert into whatever values (2);
insert into whatever values (3);
insert into whatever values (4);
insert into whatever values (5);

insert into group_table values(1, 1, null); -- champion should be B
insert into group_table values(2, 2, null); -- champion should be C
insert into group_table values(3, 3, null); -- champion should be X
insert into group_table values(4, 4, null); -- champion should be C
insert into group_table values(5, 5, null);

insert into letter values(1,1,'A');
insert into letter values(2,1,'B');
insert into letter values(3,1,'B');

insert into letter values(4,2,'B');
insert into letter values(5,2,'C');
insert into letter values(6,2,'C');

insert into letter values(7,3,'X');
insert into letter values(8,3,'X');
insert into letter values(9,3,'Y');

insert into letter values(10,4,'A');
insert into letter values(11,4,'A');
insert into letter values(12,4,'C');

insert into letter values(13,4,'C');
insert into letter values(14,4,'C');
insert into letter values(15,4,'C');

insert into letter values(16,5,'B');
insert into letter values(17,5,'C');
insert into letter values(18,5,'C');

-- update to set champions for everybody

update group_table 
join
  (select  letter,
  id_group_table, 
  count(letter) as occurences
  from letter
  group by id_group_table, letter 
  order by occurences desc
  ) tab on group_table.id_group_table = tab.id_group_table

set champion = tab.letter
where group_table.id_whatever in (1,2,3,4);

http://sqlfiddle.com/#!2/6ea061/1

谢谢！

Answer 1

documentation说：

每个匹配的行都会更新一次，即使它与条件多次匹配。

但是，并没有特别说它是第一个始终使用的值，所以我不确定你是否能够可靠地依赖它。如果你编写子查询以便它只返回每个id_group_table的冠军会更好。有很多SO问题显示如何编写这样的查询。

具有多个连接可能性的连接的MySQL更新

1 个答案: