MySQL returning an empty field: CONCAT(nonEmpty1,e

问题:

I have PHP 5 code accessing a MyISAM table on MySQL 5 server. The query looks like this:

SELECT CONCAT(fName1,' ',mName2,' ',lName3) AS userName 
    FROM users 
    WHERE level > 10

When there's no mName filled in, I am expecting output like "fname lname" , but I'm getting "" (empty string) instead (the number of rows returned is correct). Where am I making a mistake?

PHP code:

<?php
$result = mysql_query($the_above_query);
while ($result_row = mysql_fetch_assoc($result)) {
    // do stuff with the name
    // except I'm getting empty strings in $result_row['userName']
}

Relevant part of table structure:

CREATE TABLE users {
    /* -snip- */ 
    `fName1` varchar(50) default NULL,      
    `mName2` varchar(50) default NULL,      
    `lName3` varchar(50) default NULL,      
    `level` int(11) default 0,      
    /* -snip- */ 
} ENGINE=MyISAM DEFAULT CHARSET=utf8;

(also, is this way (column concatenation in MySQL) a good idea, or should I fetch the columns to PHP and join them there?)

---

Turns out that I was getting back a NULL; PHP treats a returned NULL and empty string("") similarly, you'd have to compare with === to see the difference.

回答1:

From google: http://bugs.mysql.com/bug.php?id=480

[23 May 2003 4:32] Alexander Keremidarski

Thank you for taking the time to write to us, but this is not a bug. Please double-check the documentation available at http://www.mysql.com/documentation/ and the instructions on how to report a bug at http://bugs.mysql.com/how-to-report.php

This is doccumented behaviour of CONCAT() function.

From Manual chapter 6.3.2 String Functions

CONCAT(str1,str2,...) Returns the string that results from concatenating the arguments. Returns NULL if any argument is NULL

Use CONCAT_WS() instead or wrap NULLable paremeters with IFNULL() function.

Documentation and usage for CONCAT_WS: http://dev.mysql.com/doc/refman/5.0/en/string-functions.html#function_concat-ws

回答2:

From MYSQL docs

CONCAT() returns NULL if any argument is NULL.

you want to use CONCAT_WS()

CONCAT_WS(separator,str1,str2,...)

But best bet is to just pull it back and use php cause if you need a different format or just one of those fields later you'll have to make another db call

回答3:

In MySQL concatenating any string to a NULL value results in NULL. You have to check for NULL before concatenate using IFNULL:

SELECT CONCAT(IFNULL(fName1,''),' ',IFNULL(mName2,''),' ',IFNULL(lName3,'')) AS userName 
FROM users 
WHERE level > 10

回答4:

This was the solution I came up with which included Keeper and Ersatz answer. System would not allow me to vote you guys up though :(

CONCAT_WS(IFNULL(ts_usr_nameDetails.first_name,''),' ',IFNULL(ts_usr_lib_connectionNameDetails.first_name,'')) AS composerName

This allowed for some amazing results

回答5:

This is an answer based on the solution above by @chocojosh and another question here: MySQL/SQL: Update with correlated subquery from the updated table itself

I had a similar problem, but I was trying to concat a bunch of group_concats and some were NULL which caused the whole row to be NULL. The goal was to place data from other tables into a single field in the main table to allow fulltext searches.

Here is the SQL that worked. Note the first parameter for concat_ws I made ' ', this allowed for spaces between the values for the fulltext field.

Hope this helps someone.

update
products target
INNER JOIN 
(
    select p.id, 
    CONCAT_WS(
    ' ',
        (select GROUP_CONCAT(field SEPARATOR ' ') from table1 where productId = p.id),
        p.title,' ', 
        (select GROUP_CONCAT(field, ' ', descriptions SEPARATOR ' ') from table2 where productId = p.id),
        (select GROUP_CONCAT(field SEPARATOR ' ') from table3 where productId = p.id),
        (select GROUP_CONCAT(field, ' ', catno SEPARATOR ' ') from table4 where productId = p.id),
        (select GROUP_CONCAT(field SEPARATOR ' ') from table5 where productId = p.id),
        (select GROUP_CONCAT(field SEPARATOR ' ') from table6 where productId = p.id),
        (select GROUP_CONCAT(field SEPARATOR ' ') from table7 where productId = p.id)
    ) as ft
    from products p
) as source
on target.id = source.id
set target.fulltextsearch = source.ft

回答6:

you could also the COALESCE() function to return the first non-null value. Like so:

SELECT CONCAT(fName1,COALESCE(CONCAT(' ',mName2,' '),' '),lName3) AS userName 
  FROM users 
  WHERE level > 10

This would also only put one space if there was no middle name and a space before and after if there was a middle name.

Reference for this function can be found at: http://dev.mysql.com/doc/refman/5.0/en/comparison-operators.html#function_coalesce