1、內連接：將兩個表中存在連結關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結果是一樣的（內連接的inner關鍵字可省略）；
2、外連接：分為左外連接和右外連接
左連接A、B表結果包括A的全部記錄和符合條件的B的記錄。
右聯結A、B表的結果和左聯結B、A的結果是一樣的，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執行后的結果是一樣的。
3、全聯結
4、無聯結
5、三表聯結查詢
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、終極的三表聯結查詢
items：商品表，item_visit_stats：商品訪問表，item_trade_stats：商品銷售表
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