left join 關鍵字從左表（table1）返回所有的行，即使右表（table2）中沒有匹配。如果右表中沒有匹配，則結果為 null。
sql left join 語法
select column_name(s)
from table1
left join table2
on table1.column_name=table2.column_name;
select column_name(s)
from table1
left outer join table2
on table1.column_name=table2.column_name;
select customers.customername, orders.orderid
from customers
left join orders
on customers.customerid=orders.customerid
order by customers.customername;
right join 關鍵字從右表（table2）返回所有的行，即使左表（table1）中沒有匹配。如果左表中沒有匹配，則結果為 null。
sql right join 語法
select column_name(s)
from table1
right join table2
on table1.column_name=table2.column_name;
select column_name(s)
from table1
right outer join table2
on table1.column_name=table2.column_name;
select orders.orderid, employees.firstname
from orders
right join employees
on orders.employeeid=employees.employeeid
order by orders.orderid;
full outer join 關鍵字只要左表（table1）和右表（table2）其中乙個表中存在匹配，則返回行.
full outer join 關鍵字結合了 left join 和 right join 的結果。
sql full outer join 語法
select column_name(s)
from table1
full outer join table2
on table1.column_name=table2.column_name;
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