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
        )

查询分阶段处理：

1. FROM子句和所有连接;
2. WHERE子句和所有 predicate 。因此，如果您希望在结果集中看到NULL值，您永远不应该在WHERE节中过滤OUTER连接的表列，因为这会将您的查询转换为INNER连接;
3. GROUP BY和HAVING条款;
   1.查询组合：UNION、INTERSECT、EXCEPT或MINUS
4. ORDER BY
5. LIMIT
   因此，正如其他人所指出的，在 * UNION子句之前使用ORDER BY和LIMIT * 在语法上是错误的，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;

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)

我有一个表buysell_product。我想根据views_1列选择前5个，并使用views_2列选择另一个前5个，然后合并这两个表。两个查询中的列是相同的。期望这样做能起作用，但它没有：

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:
...

它们分别工作，但我需要合并它们：

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
)

结果：
| 标识符|视图_1|视图_2|姓名|
| - -|- -|- -|- -|
| 2个|四十一|十六|优秀的2013款福特生态运动|
| 三个|七十二|10个|卓越的现代creta|
| 五个|三十九|三十九|iPhone 11 128GB|
| 七个|十二|八十四|卓越的现代creta sx|
| 九个|三十七|八十四|大众波罗1.2 GT AMT 2017款|
| 四十四|三十四|八十一|Usupso按摩防滑拖鞋|
| 四十五|十五|七十五|大蒜粉-100 Gm|
| 五十七|三十五|十一|iPhone 13和14|
| 六十七|十五|七十三|通用触摸屏电容式手写笔|