SELECT 语句中的多个记录如何在子查询语句中具有 LIMIT 1 的多个子查询

Question

SELECT
    (SELECT NAME FROM OCCUPATIONS WHERE OCCUPATION = 'Doctor'    ORDER BY NAME LIMIT 1),
    (SELECT NAME FROM OCCUPATIONS WHERE OCCUPATION = 'Professor' ORDER BY NAME LIMIT 1),
    (SELECT NAME FROM OCCUPATIONS WHERE OCCUPATION = 'Singer'    ORDER BY NAME LIMIT 1),
    (SELECT NAME FROM OCCUPATIONS WHERE OCCUPATION = 'Actor'     ORDER BY NAME LIMIT 1)
FROM OCCUPATIONS

输出显示 10 条记录，但我觉得它应该 return 1 条记录，因为我们在子查询中已经有了 LIMIT 1 条件。需要这方面的帮助！

Answer 1

如果您删除查询末尾的 FROM OCCUPATIONS，它可能会在此处提供您想要的内容：

SELECT
    (SELECT NAME FROM OCCUPATIONS WHERE OCCUPATION = 'Doctor'     ORDER BY NAME LIMIT 1), 
    (SELECT NAME FROM OCCUPATIONS WHERE OCCUPATION = 'Professor'  ORDER BY NAME LIMIT 1),
    (SELECT NAME FROM OCCUPATIONS WHERE OCCUPATION = 'Singer'     ORDER BY NAME LIMIT 1),
    (SELECT NAME FROM OCCUPATIONS WHERE OCCUPATION = 'Actor'      ORDER BY NAME LIMIT 1);

但是，在 MySQL 中执行此操作的更典型方法是通过聚合：

SELECT o1.NAME
FROM OCCUPATIONS o1
INNER JOIN
(
    SELECT OCCUPATION, MIN(NAME) AS NAME
    FROM OCCUPATIONS
    GROUP BY OCCUPATION
) o2
    ON o2.OCCUPATION = o1.OCCUPATION AND
       o2.NAME = o1.NAME;

如果您使用的是 MySQL 8+，那么 ROW_NUMBER 提供了另一种方法：

WITH cte AS (
    SELECT *, ROW_NUMBER() OVER (PARTITION BY OCCUPATION ORDER BY NAME) rn
    FROM OCCUPATIONS
)

SELECT NAME
FROM cte
WHERE rn = 1;

Answer 2

使用聚合！

SELECT MIN(CASE WHEN OCCUPATION = 'Doctor' THEN NAME END),
       MIN(CASE WHEN OCCUPATION = 'Professor' THEN NAME END),
       MIN(CASE WHEN OCCUPATION = 'Singer' THEN NAME END),
       MIN(CASE WHEN OCCUPATION = 'Actor' THEN NAME END)
FROM OCCUPATIONS;

这似乎是表达逻辑的最简单方式。

您的查询为 OCCUPATIONS 中的每一行返回一行，因为查询没有过滤或聚合。每行恰好具有相同的值，因为除了 FROM 子句之外，查询中的任何地方都没有使用外部 table。

条件聚合（如上所示）是表达这种逻辑的一种更简单的方式。

SELECT 语句中的多个记录如何在子查询语句中具有 LIMIT 1 的多个子查询

How are the multiple number of records in a SELECT statement with multiple subqueries having LIMIT 1 in the subquery statements

sql

select

limit