右聯(lián)結(jié)A舆床、B表的結(jié)果和左聯(lián)結(jié)B侨拦、A的結(jié)果是一樣的只怎，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)果是一樣的梳凛。
3磕潮、全聯(lián)結(jié)
4公罕、無聯(lián)結(jié)
5器紧、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
items：商品表楼眷，item_visit_stats：商品訪問表铲汪，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
