mysql内联接、左联接、右联接

--内联接----
select goods_id,goods_name,cate_name from tdb_goods inner join tdb_goods_cates on tdb_goods.cate_id=tdb_goods_cates.cate_id\G; 内联接返回两张表中都有的
select goods_id,goods_name,brand_name from tdb_goods inner join tdb_brand on tdb_goods.brand_id=tdb_brand.brand_id;
--左连接--
select goods_id,goods_name,cate_name from tdb_goods left join tdb_goods_cates on tdb_goods.cate_id=tdb_goods_cates.cate_id\G; 左联接返回第一张（即左边）表中都有的
select goods_id,goods_name,brand_name from tdb_goods left join tdb_brand on tdb_goods.brand_id=tdb_brand.brand_id;             所参照右表没有的值用null代替
--右连接--
select goods_id,goods_name,cate_name from tdb_goods right join tdb_goods_cates on tdb_goods.cate_id=tdb_goods_cates.cate_id\G; 右联接返回第二张（即右边）表中都有的
select goods_id,goods_name,brand_name from tdb_goods right join tdb_brand on tdb_goods.brand_id=tdb_brand.brand_id;             所参照左表没有的值用null代替
对照：select goods_id,goods_name,brand_name from tdb_brand left join tdb_goods on tdb_goods.brand_id=tdb_brand.brand_id;

总结：内联接只会显示两张表中都有的列，左连接参照左表，右表中无值就设null，右连接参照右表，左表无值就设null，左右连接可互换表名结果相同。