如果子查询的结果为 NULL，MySQL 整个查询将失败

Posted 2023-02-24

【中文标题】如果子查询的结果为 NULL，MySQL 整个查询将失败

【英文标题】：MySQL the whole query fails if result of sub query is NULL

【发布时间】：2013-12-23 15:52:36

【问题描述】：

我有一种情况，我需要从同一张表的有效数据中生成包含随机数据的行

我已经通过$min = 1 和$max = [SELECT MAX(ID) FROM patient] - 1 的php rand($min, $max) 函数生成ID

Select

tblFirstName.FirstName,
tblLastName.LastName,
tblBirthDate.BirthDate,
tblLocation.Location

From

(SELECT FirstName FROM patient WHERE ID > 11445 AND FirstName != '' LIMIT 1) AS tblFirstName,
(SELECT LastName FROM patient WHERE ID > 74964 AND LastName != '' LIMIT 1) AS tblLastName,
(SELECT BirthDate FROM patient WHERE ID > 26360 LIMIT 1) AS tblBirthDate,
(SELECT Location FROM patient WHERE ID > 68356 AND Location != '' LIMIT 1) AS tblLocation

现在在上述查询的ID > 26360 中，26360 是随机数，> 用于避免删除 26360 的可能性

问题：

如果任何子查询没有返回结果，则完整查询失败并且什么也不返回。

【参考方案1】：

试试这个：

SELECT (SELECT FirstName FROM patient WHERE ID > 11445 AND FirstName != '' LIMIT 1) AS tblFirstName,
         (SELECT LastName FROM patient WHERE ID > 74964 AND LastName != '' LIMIT 1) AS tblLastName,
         (SELECT BirthDate FROM patient WHERE ID > 26360 LIMIT 1) AS tblBirthDate,
         (SELECT Location FROM patient WHERE ID > 68356 AND Location != '' LIMIT 1) AS tblLocation

【参考方案2】：

将所有查询更改为：

SELECT FirstName FROM patient WHERE ID > 11445 AND FirstName != '' UNION SELECT NULL FROM DUAL LIMIT 1

即添加 UNION SELECT NULL FROM DUAL（在所有子查询中）

【参考方案3】：

你需要使用左外连接：

Select

tblFirstName.FirstName,
tblLastName.LastName,
tblBirthDate.BirthDate,
tblLocation.Location

From

(SELECT FirstName FROM patient WHERE ID > 11445 AND FirstName != '' LIMIT 1) AS tblFirstName
LEFT JOIN
(
(SELECT LastName FROM patient WHERE ID > 74964 AND LastName != '' LIMIT 1) AS tblLastName, (SELECT BirthDate FROM patient WHERE ID > 26360 LIMIT 1) AS tblBirthDate,
(SELECT Location FROM patient WHERE ID > 68356 AND Location != '' LIMIT 1) AS tblLocation
)