如何使用三个表的连接

Question

I have 3 tables. I want to fetch the employee details for those employees only in table3. But when i ran the query i am getting all the employees names from table1

Table1 and table 2 has many employees.

Table3 has only one employee name.

Is the below query is correct? need some advise !!

i am using left join because i want mis-match records also from table1

select emp.emp_name, emp.dept, sal.salary 
from table1 as emp 
   left join table2 as sal on emp.emp_name = sal.emp_name 
                          and emp.emp_name in (select emp_name from table3)

Answer 1

Your query is not correct, given what you want it to do, as the and emp.emp_name in ... part is part of the left join condition and won't limit the rows in table1 . This is why you get all rows from table1 .

To get all rows in table1 that have matches in table3 , plus the salary stuff (or null if missing) from table2 you can either change the second part of the left join condition to a where clause or use an inner join with table3 :

-- using a where clause
select emp.emp_name, emp.dept, sal.salary 
from table1 as emp 
left join table2 as sal on emp.emp_name = sal.emp_name 
where emp.emp_name in (select emp_name from table3);

-- or an inner join
select emp.emp_name, emp.dept, sal.salary 
from table1 as emp 
join table3 as t3 on emp.emp_name = t3.emp_name
left join table2 as sal on emp.emp_name = sal.emp_name;

For example if your tables had the following data:

table1: emp1, emp2, emp3
table2: emp1, emp2
table3: emp1, emp3

you would get:

emp_name   salary
emp1       1000
emp3       NULL

Answer 2

Do Inner Join to get correct result

 select 
   emp.emp_name, 
   emp.dept, 
   sal.salary 
 from table1 as emp 
   inner join table2 as sal on emp.emp_name = sal.emp_name and 
                               emp.emp_name in (select emp_name from table3)