MYSQL Union All 不通过两个合并查询返回所有记录

Posted 2023-03-24

【中文标题】MYSQL Union All 不通过两个合并查询返回所有记录

【英文标题】：MYSQL Union All not returning all record by two merging query

【发布时间】：2020-12-08 06:08:53

【问题描述】：

SELECT tah.project_id_new as "project_id", tah.history_createdon from task_audit_history tah 
where task_id=64 and project_id != project_id_new;

Result

SELECT tah.project_id as "project_id", tah.history_createdon from task_audit_history tah 
where task_id=64 ORDER BY history_createdon asc limit 1;

Result

When I tried to combined above two queries using UNION ALL it return only one record. Why ? i mean it 
should include all record from two query.

SELECT tah.project_id_new as "project_id", tah.history_createdon from task_audit_history tah 
where task_id=64 and project_id != project_id_new UNION ALL SELECT tah.project_id as "project_id", tah.history_createdon from task_audit_history tah 
where task_id=64 ORDER BY history_createdon asc limit 1;

Query result

【问题讨论】：

显示第三个查询的精确 SQL 文本。

下次请将结果作为文本粘贴到此处，而不是链接到图像。在这里阅读原因：meta.***.com/questions/285551/…

【参考方案1】：

您的结果意味着 ORDER BY 和 LIMIT 子句被应用于整个联合查询，而不仅仅是子查询（这是您想要的行为）。尝试使用这个版本：

(SELECT tah.project_id_new AS project_id, tah.history_createdon
 FROM task_audit_history tah 
 WHERE task_id = 64 AND project_id != project_id_new)
UNION ALL
(SELECT tah.project_id AS project_id, tah.history_createdon
 FROM task_audit_history tah 
 WHERE task_id = 64
 ORDER BY history_createdon
 LIMIT 1);

为了完整起见，以下查询可能是您最初运行的结果，结果可能只有一行：

SELECT tah.project_id_new AS project_id, tah.history_createdon
FROM task_audit_history tah 
WHERE task_id = 64 AND project_id != project_id_new
UNION ALL
SELECT tah.project_id AS project_id, tah.history_createdon
FROM task_audit_history tah 
WHERE task_id = 64
ORDER BY history_createdon
LIMIT 1;

在这种情况下，ORDER BY 和 LIMIT 子句适用于整个联合查询，这意味着它将只返回一条记录。