SQL:元组比较 [英] SQL: tuple comparison

问题描述

在我当前的应用程序中，我需要能够执行这种类型的查询:

In my current application, I need to be able to do this type of query:

SELECT MIN((colA, colB, colC)) 
FROM mytable
WHERE (colA, colB, colC) BETWEEN (200, 'B', 'C') AND (1000, 'E', 'F')

，并在给出以下数据的情况下获得(333, 'B', 'B')的答案:

and get the answer of (333, 'B', 'B'), given this data:

+------+------+------+
| colA | colB | colC |
+------+------+------+
|   99 | A    | A    |
|  200 | A    | Z    |
|  200 | B    | B    |
|  333 | B    | B    |
|  333 | C    | D    |
|  333 | C    | E    |
|  333 | D    | C    |
| 1000 | E    | G    |
| 1000 | F    | A    |
+------+------+------+

在真正的SQL中最有效的方法是什么?请记住，这是一个示例，我的实际应用程序中的表具有不同的列和数据类型以及数亿行.我使用MySQL，如果有帮助的话.您还可以假定这些列上具有PRIMARY或UNIQUE索引.

What is the most efficient way to accomplish this in real SQL? Please keep in mind that this is a toy example, and that my actual application has tables with varying columns and data types, and hundreds of million of rows. I use MySQL, if that helps. You can also assume that these columns have a PRIMARY or UNIQUE index on them.

如果该解决方案可以轻松扩展到更多/更少的列，那就更好了.

If the solution is easily extensible to more/less columns, that's even better.

元组比较:

几个人问过，所以我应该把这个放在问题中.元组按字典顺序排序，这意味着序列与其第一个不同元素的排序相同.例如，(1,2，x)< (1,2，y)返回的值与x< y.

Several have asked so I should put this in the question. Tuples are ordered lexicographically, meaning that the sequences are ordered the same as their first differing elements. For example, (1,2,x) < (1,2,y) returns the same as x < y.

值得注意的是，SQL(或至少是mysql)正确地实现了这一点:

It's worth noting that SQL (or at least mysql) implements this correctly:

mysql> select (200, 'B', 'C') < (333, 'B', 'B') and (333, 'B', 'B') < (1000, 'E', 'F');
+--------------------------------------------------------------------------+
| (200, 'B', 'C') < (333, 'B', 'B') and (333, 'B', 'B') < (1000, 'E', 'F') |
+--------------------------------------------------------------------------+
|                                                                        1 |
+--------------------------------------------------------------------------+
1 row in set (0.00 sec)

---

以下是创建示例的必要SQL:

---

Here's the necessary SQL to create the example:

create table mytable select 333 colA, 'B' colB, 'B' colC;
insert into mytable values (200, 'B', 'B'), (333, 'C', 'D'), (1000, 'E', 'G'), 
    (200, 'A', 'Z'), (1000, 'F', 'A'), (333, 'C', 'E'), (333, 'D', 'C'),
    (99, 'A', 'A');
alter table mytable add unique index myindex (colA, colB, colC);

添加此索引似乎会使表按字典顺序排序，这很有趣.在我们的生产系统中并非如此.

Adding this index seems to cause the table to be sorted lexicographically, which is interesting. This isn't true in our production system.

推荐答案

只需:

SELECT colA
     , colB
     , colC
FROM mytable
WHERE ( ('A',  'B',  'C') <= (colA, colB, colC ) )
  AND ( (colA, colB, colC) <= ('D',  'E',  'F' ) )
ORDER BY colA, colB, colC
LIMIT 1
;

效果很好.而且我怀疑它也应该很快.

It works just fine. And I suspect is should be pretty fast, too.

这是等效的，但它可能具有更好的性能，具体取决于您的表:

This is equivalent but it may have better performance, depending on your tables:

SELECT m.colA
     , m.colB
     , m.colC
FROM mytable m
WHERE ( ('A',  'B',  'C') <= (m.colA, m.colB, m.colC) )
  AND ( (m.colA, m.colB, m.colC) <= ('D',  'E',  'F') )
  AND NOT EXISTS
  ( SELECT 1
    FROM mytable b
    WHERE (b.colA, b.colB, b.colC) < (m. colA, m.colB, m.colC)
      AND ( ('A',  'B',  'C') <= (b.colA, b.colB, b.colC) )
  );

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