Not exists with concatenated columns

[niche]

This code works with cols that are not concatenated:

 

SELECT  
		item3 
	FROM    
		nconnectz t1
	WHERE   
		NOT EXISTS
			(
				SELECT  
					item3
				FROM    
					itemandsize t2
				WHERE   
					t1.item3 = t2.item3
        )
	GROUP BY item3

I think this is the same query with concatenated columns

 

SELECT  
		CONCAT_WS(',', itemandsize, pkg_quantity) item4 
	FROM    
		nconnectz t1
	WHERE   
		NOT EXISTS
			(
				SELECT  
					CONCAT_WS(',', itemandsize, '0.00') item4
				FROM    
					itemandsize t2
				WHERE   
					t1.item4 = t2.item4
        )
	GROUP BY item4

the error is: Unknown column 't1.item4' in 'where clause'

what's causing the error?

[niche]

Voila !

 

SELECT  
		nconnectz.itemandsize, nconnectz.pkg_quantity
	FROM    
		nconnectz  
	WHERE   
		NOT EXISTS
			(
				SELECT  
					*
				FROM    
					itemandsize  
				WHERE   
					CONCAT_WS(',', nconnectz.itemandsize, nconnectz.pkg_quantity)  = CONCAT_WS(',', itemandsize.itemandsize, '0.00')
        )