与联合所有使用mysql问题

Question

 select `C1`.`ID` AS `ID`,
`C1`.`Article_ID` AS `Article_ID`,
`C1`.`DateAdded` AS `DateAdded`,
`C1`.`Votes` AS `Votes`,
`C1`.`Content` AS `Content`,
(select 0) AS `isReply`,
`C1`.`Usr_ID` AS `Usr_ID`,
(select NULL) AS `RepliedTo` 
from `Comment` as C1
where isnull(`C1`.`Reply_ID`) limit 5
UNION ALL
select 
`C2`.`Reply_ID` AS `Reply_ID`,
`C2`.`Article_ID` AS `Article_ID`,
`C2`.`DateAdded` AS `DateAdded`,
`C2`.`Votes` AS `Votes`,
`C2`.`Content` AS `Content`,
`C2`.`ID` AS `isReply`,
`C2`.`Usr_ID` AS `Usr_ID`,
`C2`.`RepliedTo` AS `RepliedTo` 
from `Comment` as C2
where (`C2`.`Reply_ID` = `C1`.`Reply_ID` )

My issue is in the last line where (C2.Reply_ID = C1.Reply_ID ) in specific the C2.Reply_ID=C1.Reply_ID

an Error with Unknown column C1.Reply_ID in where clause is popping out.

My Objective is that I want to Retrieve

All rows of Table1

Union All

The rows of Table2 using the ID's retrieved from Table1 in where clause

Answer 1

You can not use the tables from the first select in the second select.

You have to repeat your first query in the second select query

select `C1`.`ID` AS `ID`,
`C1`.`Article_ID` AS `Article_ID`,
`C1`.`DateAdded` AS `DateAdded`,
`C1`.`Votes` AS `Votes`,
`C1`.`Content` AS `Content`,
(select 0) AS `isReply`,
`C1`.`Usr_ID` AS `Usr_ID`,
(select NULL) AS `RepliedTo` 
from `Comment` as C1
where isnull(`C1`.`Reply_ID`) limit 5
UNION ALL
select 
`C2`.`Reply_ID` AS `Reply_ID`,
`C2`.`Article_ID` AS `Article_ID`,
`C2`.`DateAdded` AS `DateAdded`,
`C2`.`Votes` AS `Votes`,
`C2`.`Content` AS `Content`,
`C2`.`ID` AS `isReply`,
`C2`.`Usr_ID` AS `Usr_ID`,
`C2`.`RepliedTo` AS `RepliedTo` 
from `Comment` as C2
where (`C2`.`Reply_ID` in (select `C1`.`ID` 
from `Comment` as C1
where isnull(`C1`.`Reply_ID`) limit 5)

Answer 2

This should give you close to what you want:

SELECT `ID` AS `ID`
   , `Article_ID` AS `Article_ID`
   , `DateAdded` AS `DateAdded`
   , `Votes` AS `Votes`
   , `Content` AS `Content`
   , IF(copyTbl.copyNum=1, 0, `ID`) AS `isReply`
   , `Usr_ID` AS `Usr_ID`
   , IF(copyTbl.copyNum=1, NULL, `RepliedTo`) AS `RepliedTo` 
FROM `Comment`, (SELECT 1 AS copyNum UNION SELECT 2) AS copyTbl
WHERE `Reply_ID` IS NULL
ORDER BY `ID`, copyTbl.copyNum
LIMIT 10
;

It assumes Comment . ID is unique; if that is not the case, you'll need to adjust the ORDER BY accordingly.