1、內(nèi)連接：將兩個(gè)表中存在連結(jié)關(guān)系的字段符合連接條件的記錄形成記錄集

Select A.name,B.name from A inner join B on A.id=B.id和

Select A.name,B.name from A,B where A.id=B.id結(jié)果是一樣的（內(nèi)連接的inner關(guān)鍵字可省略）；

2、外連接：分為左外連接和右外連接

左連接A、B表結(jié)果包括A的全部記錄和符合條件的B的記錄。

右聯(lián)結(jié)A、B表的結(jié)果和左聯(lián)結(jié)B、A的結(jié)果是一樣的，也就是說：

Select A.name,B.name from A Left Join B on A.id=B.id和

Select A.name,B.name from B Right Join A on B.id-A.id執(zhí)行后的結(jié)果是一樣的。

3、全聯(lián)結(jié)

4、無聯(lián)結(jié)

5、三表聯(lián)結(jié)查詢

select username,psw,gname,tel from (t1 left join t2 on t1.t1_id=t2.t1_id) left join t3 on t1.t1_id=t3.t1_id

6、終極的三表聯(lián)結(jié)查詢

SELECT i.num_iid, i.title, i.price, SUM(iv.user_visits) AS uv,it.buyer_num,it.item_num,it.item_num*i.price AS turnover
FROM (items AS i RIGHT JOIN item_visit_stats AS iv ON i.num_iid=iv.num_iid)
LEFT JOIN (SELECT num_iid,SUM(buyer_num) AS buyer_num,SUM(item_num) AS item_num FROM item_trade_stats
WHERE seller_nick="XXXX" AND business_day BETWEEN '2010-08-14' AND '2010-08-15' GROUP BY num_iid)
AS it ON it.num_iid=iv.num_iid
WHERE i.nick="XXXX" AND iv.business_day BETWEEN '2010-08-14' AND '2010-08-15'
GROUP BY i.num_iid ORDER BY uv DESC