MySQL查询选择/连接concat错误

Question

What is the correct way to perform this query? Additionally, I'd like to trim spaces from fullname, because if middle is empty it still returns the spaces before and after it.

SELECT first,middle,last, 
CONCAT(first,' ',middle,' ',last) AS fullname
FROM names a 
LEFT JOIN info b ON fullname = b.name
LIMIT 1

the current error is: ERROR 1054 (42S22): Unknown column 'fullname' in 'on clause'

Answer 1

The problem that you are facing is that you are trying to join on a calculated column.

You need to first calculate the column in a subselect, and then join to that 'table'

Something like

SELECT  *
FROM    (
            SELECT  first,
                    middle,
                    last,  
                    CONCAT(first,' ',middle,' ',last) AS fullname 
            FROM    names 
        ) a  LEFT JOIN 
        info b  ON  fullname = b.name 
LIMIT 1 

You can also have a look at the CASE Statement for the spaces in the middle.

Answer 2

You can't use an alias in the ON clause. Try doing

SELECT first,middle,last,
CONCAT(first,' ',middle,' ',last) AS fullname
FROM names a 
LEFT JOIN info b ON b.name=CONCAT(first,' ',middle,' ',last) 

"The conditional_expr used with ON is any conditional expression of the form that can be used in a WHERE clause" (from dev.mysql.com/doc/refman/5.1/en/join.html).

So as a logical inference you're not allowed to use aliases in ON clauses.

Answer 3

Try this:

select first, last, CONCAT(first,' ',middle,' ',last) as fullNAME from names a left join info b on (CONCAT(first,' ',middle,' ',last) =b.name)

one more thing, u have skipped a comma after last in ur select query

Answer 4

try this ,

SELECT CONCAT(first,' ',middle,' ',last) AS fullname FROM names a

LEFT JOIN info b

ON fullname = b.name

LIMIT 1