table1與table2的等值連線如下：Select A.id,A.name,A.housenumber ,B.housenumber,B.housename from table1 A ,table2 B where A.housenumber = B.housenumber                                                                       A.housenumber 與 B.housenumber分別是外來鍵和主鍵，如果A.housenumber有允許有空 值，當你想選擇TABLE1的所有列，等值連線使得資料丟失。                                            比如：

 裝置表A中多條裝置記錄，連線到B表的供應商記錄，我們想將擁有所有的裝置列出，就應該選擇 A左外連線B，它顯示A中的所有行，並連線到B中的供應商資訊。  A表左外連線B表的SQL語句：

              select A.id,A.name,B.supply from table1 A left outer join table2 B  on A.supply_id = B.supply_id         

A表左外連線B，C,D,E等多表並且由選擇條件輸出的SQL語句：

 select A.machine_id,A.pro_num,A.machine_name ,A.machine_type,B.brand_name,A.unitprice,A.sn,A.service_num,C.supply_name,D.room_name,A.install_place,A.install_man,E.state_name,A.buydate,F.project_name ,A.memo  from machine A left outer join brand  B on (A.brand_id = B.brand_id)
 left  outer join supply   C on (A.supply_id = C.supply_id) 
 left   outer  join room D on (A.room_id = D.room_id)
 left  outer join  state  E on (A.state = E.state_id)
 left   outer join project F on (A.project_id = F.project_id)
where (A.machine_id = 7)

專門講外連線的