使用 UNION 将两个带有 LIMITS 的语句组合起来答案

【问题标题】：Combine two statements with LIMITS using UNION使用 UNION 将两个带有 LIMITS 的语句组合起来
【发布时间】：2022-11-16 14:12:06
【问题描述】：

有没有办法将这两个语句合并为一个而不会出现重复条目？

SELECT * FROM Seq where JULIANDAY('2012-05-25 19:02:00')<=JULIANDAY(TimeP) 
         order by TimeP limit 50

SELECT * FROM Seq where JULIANDAY('2012-05-29 06:20:50')<=JULIANDAY(TimeI)  
         order by TimeI limit 50

SQLITE 不支持我的第一个明显尝试（语法错误：Limit 子句应该在 UNION 之后而不是之前）：

SELECT * FROM Seq where JULIANDAY('2012-05-25 19:02:00')<=JULIANDAY(TimeP) 
         order by TimeP limit 50
UNION
SELECT * FROM Seq where JULIANDAY('2012-05-29 06:20:50')<=JULIANDAY(TimeI)
         order by TimeI limit 50

【问题讨论】：

标签： sql sqlite

【解决方案1】：

使用子查询并在其中执行限制。

SELECT  *
FROM    (   SELECT  * 
            FROM    Seq 
            WHERE   JULIANDAY('2012-05-25 19:02:00') <= JULIANDAY(TimeP) 
            ORDER BY TimeP
            LIMIT 50
        )
UNION
SELECT  *
FROM    (   SELECT  * 
            FROM    Seq 
            WHERE   JULIANDAY('2012-05-29 06:20:50') <= JULIANDAY(TimeI) 
            ORDER BY TimeI
            LIMIT 50
        )

【讨论】：

【解决方案2】：

查询分阶段处理：

FROM 子句和所有连接；
WHERE 子句和所有谓词。所以如果你想在结果集中看到NULL值，你永远不应该在WHERE部分过滤OUTER-joined表列，因为这会将你的查询变成INNERjoin；
GROUP BY和HAVING条款；
查询组合：UNION、INTERSECT、EXCEPT或MINUS
ORDER BY

LIMIT

因此，正如其他人指出的那样，使用 ORDER BY 和 LIMIT 在语法上是错误的前UNION 条款。您应该使用子查询：

SELECT *
  FROM (SELECT * FROM Seq
         WHERE JULIANDAY('2012-05-25 19:02:00') <= JULIANDAY(TimeP) 
         ORDER BY TimeP LIMIT 50) AS tab1
UNION
SELECT *
  FROM (SELECT * FROM Seq
         WHERE JULIANDAY('2012-05-29 06:20:50') <= JULIANDAY(TimeI)  
         ORDER BY TimeI LIMIT 50) AS tab2;

【讨论】：

【解决方案3】：

           SELECT * from 
           (SELECT * 
           FROM Seq 
           where JULIANDAY('2012-05-25 19:02:00')<=JULIANDAY(TimeP) 
           order by TimeP limit 50)
           UNION
           SELECT * from 
           (SELECT * 
           FROM Seq 
           where JULIANDAY('2012-05-29 06:20:50')<=JULIANDAY(TimeI)
           order by TimeI limit 50)

这应该可以解决问题

【讨论】：

【解决方案4】：

我有一个 SQL（实际上是 SQLite）数据库，其中有一个名为 buysell_product 的表。我想要做的是根据views_1 列选择前 5 名，然后再次使用 views_2 列选择前 5 名。然后合并两个选择。两个查询中选择的所有列都相同。

我期待下面的工作，但它没有。

询问

SELECT id, name, views_1, views_2
    FROM buysell_product
    ORDER BY views_1 DESC
    LIMIT 5
UNION
SELECT id, name, views, views_2
    FROM buysell_product as b
    ORDER BY views_2 DESC
    LIMIT 5

错误

Execution finished with errors.
Result: ORDER BY clause should come after UNION not before
At line 1:
SELECT id, name, views_1, views-2
    FROM buysell_product
    ORDER BY views_1 DESC
    LIMIT 5
UNION
SELECT id, name, views_1, views_2
    FROM buysell_product as b
    ORDER BY views_2 DESC
    LIMIT 5

而且，我尝试了以下有效的方法。

查询 1

    SELECT id, views_1, name
    FROM buysell_product
    ORDER BY views_1 DESC
    LIMIT 5

查询 2

SELECT id, views_2, name
    FROM buysell_product
    ORDER BY views_2 DESC
    LIMIT 5

但是，我们需要通过以下方式将上述两个查询合并为一个：

SELECT *
    FROM (
        SELECT id, views_1, views_2, name
        FROM buysell_product
        ORDER BY views_1 DESC
        LIMIT 5
        )
UNION
SELECT * 
    FROM (
        SELECT id, views_1, views_2, name
        FROM buysell_product as b
        ORDER BY views_2 DESC
        LIMIT 5
        )

输出

id  views_1 views_2 name
2   41  16  Excellent 2013 ford ecosport
3   72  10  Excellent Hyundai creta 
5   39  39  iPhone 11 128gb
7   12  84  Excellent Hyundai creta sx
9   37  84  Volkswagen Polo 1.2 GT AMT 2017
44  34  81  Usupso Massage Anti Skid Slippers
45  15  75  Garlic Powder - 100Gm
57  35  11  Iphone 13 and 14
67  15  73  Universal Touch Screen Capacitive Stylus

【讨论】：