一、表之間的連線（左、右、內、外連線）

左連線（右表只顯示兩個表中都具有相同a.emp_id = s.hjempid,左表全部列出）

select * from employeehj a  left join employeehj1 s on a.emp_id = s.hjempid
select * from employeehj a, employeehj1 s where  a.emp_id = s.hjempid(+)
右連線（左表只顯示兩個表中都具有相同a.emp_id = s.hjempid,右表全部列出）
select * from employeehj a  right join employeehj1 s on a.emp_id = s.hjempid
select * from employeehj a , employeehj1 s where a.emp_id(+) = s.hjempid
內連線(只顯示兩個表中都具有相同a.emp_id = s.hjempid)
select * from employeehj a inner join employeehj1 s on a.emp_id = s.hjempid
外連結(兩個表中資料全部顯示出來，如果沒有匹配到另一張表中得資料，就與NULL匹配)
select * from employeehj a full  join employeehj1 s on a.emp_id = s.hjempid

select * from employeehj a full outer join employeehj1 s on a.emp_id = s.hjempid

二、case when else end

select  t.class ,sum(case )