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I have this query that return invoices made on a certain employee shift

SELECT

i.dateTime,i.amount,i.totalProfit,i.shiftID,

i_o.itemID,i_o.quantity,

item.name itemName,

p.full_name



from invoice i 



LEFT JOIN 

inv_order i_o on i_o.invID=i.invID



LEFT JOIN 

`item-service` item on item.itemID = i_o.itemID





LEFT JOIN person p on 

p.PID=i.personID



where i.shiftID =97

but then, i need to get the employee name from the employee table, and i only have the shiftID.

SELECT

i.dateTime,i.type,i.amount,i.totalProfit,i.shiftID,i_o.itemID,i_o.quantity,item.name itemName,p.full_name



from invoice i 



LEFT JOIN 

inv_order i_o on i_o.invID=i.invID



LEFT JOIN 

`item-service` item on item.itemID = i_o.itemID





LEFT JOIN person p on 

p.PID=i.personID



where i.shiftID =97



UNION



SELECT e.name from employee e



left join shift s on s.empID = e.empID



where s.shiftID =97

the mysql return this error

The used SELECT statements have a different number of columns

The error message is quite clear as to the problem: your first query returns 8 columns of data and your second only 1, so you can't UNION them. It seems you probably want to just JOIN the employee table via the shift table e.g.

SELECT i.dateTime

      ,i.type

      ,i.amount

      ,i.totalProfit

      ,i.shiftID

      ,i_o.itemID

      ,i_o.quantity

      ,item.name AS itemName

      ,p.full_name

      ,e.name

FROM invoice i 

LEFT JOIN inv_order i_o ON i_o.invID=i.invID

LEFT JOIN `item-service` item ON item.itemID = i_o.itemID

LEFT JOIN person p ON p.PID=i.personID

LEFT JOIN shift s ON s.shiftID = i.shiftID

LEFT JOIN employee e ON e.empID = s.empID

WHERE i.shiftID = 97