Mysql 如果有多个同名列，请选择非空列

我有两张桌子

abc has the following columns  "value","timeStamp"
def has the columns "value","timeStamp"<

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我如何实现这一点

您可以使用如下的CASE表达式实现这一点：

select abc.value as `ABC Value`,
       def.value as `DEF Value`,
       case when abc.timeStamp is not null then abc.timeStamp else def.timeStamp as `timeStamp`
from abc,def;

我假设您希望通过公共列值连接此表（并创建mysql中不存在的完整外部连接，因此需要两个左连接），因此您应该有一个连接条件（我还将更改隐式连接语法为正确的连接语法）

使用coalesce（）函数获得所需的结果。请注意，如果两个时间戳列都为null，则它将返回null：

select abc.value as `ABC Value`,def.value as `DEF Value`,
       coalesce( abc.timeStamp, def.timeStamp) as ts
from abc,def;

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你应该编辑你的问题，提供样本数据和期望的结果。我不认为你可以在这样的选择中使用if，我很确定你需要使用case语句

if

只能在您正在查找的MySQL中的存储过程/函数中使用

case

语法。什么是“期望结果”？按特定顺序从多个列中选择一个非空列。

select abc.value as `ABC Value`,
       def.value as `DEF Value`,
       case when def.timeStamp is not null then def.timeStamp else abc.timeStamp as `timeStamp`
from abc  
LEFT OUTER JOIN def ON(abc.value = def.value)
UNION
select abc.value as `ABC Value`,
       def.value as `DEF Value`,
       case when abc.timeStamp is not null then abc.timeStamp else def.timeStamp as `timeStamp`
from def 
LEFT OUTER JOIN abc ON(abc.value = def.value)

select abc.value as `ABC Value`,def.value as `DEF Value`,
       coalesce( abc.timeStamp, def.timeStamp) as ts
from abc,def;