MYSQL unknown clause join column in next join

问题

I have the following query:

SELECT * FROM questions 
LEFT JOIN answers ON (questions.id = answers.id AND (connections.username = answers.username OR connections.username = 'bob' OR answers.username IS NULL))
LEFT JOIN connections ON connections.username1 = 'mikha' AND
                         (connections.username2 = answers.username) 
LEFT JOIN answers answers2 ON (questions.id = answers2.id)
WHERE (answers.id <> answers2.id)

I get the following error:

`Unknown connections.username in ON clause`

I define some conditions in the first join and want to get the rest that don't match these conditions. That's why I use this part answers.id AND answers2.id. To get IDs that don't match the conditions in the first left join.

Thanks

回答1:

Try it like this, I have no schema to test it myself but I feel like it should work(or something like this)

SELECT * FROM questions, connections
LEFT JOIN answers ON (questions.id = answers.id AND
                     connections.username2 = answers.username)
where connections.username1 = 'mikha';

eventually like this

SELECT * FROM questions
LEFT JOIN answers ON (questions.id = answers.id)
LEFT JOIN connections ON (connections.username2 = answers.username)
where connections.username1 = 'mikha';

EDIT: I found this in documentation

Example:

CREATE TABLE t1 (i1 INT); CREATE TABLE t2 (i2 INT); CREATE TABLE t3 (i3 INT); SELECT * FROM t1 JOIN t2 ON (i1 = i3) JOIN t3;

Previously, the SELECT statement was legal. Now the statement fails with an Unknown >column 'i3' in 'on clause' error because i3 is a column in t3, which is not an operand of >the ON clause. The statement should be rewritten as follows:

SELECT * FROM t1 JOIN t2 JOIN t3 ON (i1 = i3);

So for Your case it may be

SELECT * FROM questions  
LEFT JOIN connections
LEFT JOIN answers ON (connections.username1 = 'mikha' AND questions.id = answers.id AND
                 connections.username2 = answers.username)

回答2:

Effectively if your limiting the results of answers by the results of connections you are doing an inner join not two left joins because one row cannot return with out results from the other because null does not equal null. Given this to be true you can use the following because I have no clue what your schema is all schema was made up to protect the innocent.

SELECT * FROM questions 
left join (
    select connections.otherstuff,answers.id,answers.text from connections 
    inner join answers ON  connections.username2 = answers.username
    where connections.username1 = 'mikha'
 ) conn on questions.id = conn.id

http://sqlfiddle.com/#!2/c7252/1

来源：https://stackoverflow.com/questions/13783669/mysql-unknown-clause-join-column-in-next-join

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mysql

sql

database

clause