use the If else condition for selecting the column in mysql?

Question

I am having two tables like this.Both are separate tables

AccountNo       User         Name
----------------------------------
 1               U            a
 2               U            b
 3               U            c

And another table contains the following structure

 TempAccountNo       Mycolumn    AccountNo     
------------------------------------------
 4               X                2341
 5               Y                 2
 6               Z                2568

I need to select the AccountNo or TempAccountNo,Mycolumn From table II and the condition is

If (tableII.AccountNo not in table I)

I need to choose TempAccountNo from table II

else

I need to choose AccountNo from table II

How can I achieve this.

Answer 1

   SELECT IF(t1.AccountNo IS NULL, t2.TempAccountNo, t2.AccountNo) AS optional_field
     FROM table2 t2
LEFT JOIN t1 ON t1.AccountNo = t2.AccountNo