0. 準備相關表來進行接下來的測試
相關建表語句請看：https://github.com/YangBaohust/my_sql
user1表，取經組+----+-----------+-----------------+---------------------------------+| id | user_name | comment | mobile |+----+-----------+-----------------+---------------------------------+| 1 | 唐僧 | 旃檀功德佛 | 138245623,021-382349 || 2 | 孫悟空 | 斗戰勝佛 | 159384292,022-483432,+86-392432 || 3 | 豬八戒 | 凈壇使者 | 183208243,055-8234234 || 4 | 沙僧 | 金身羅漢 | 293842295,098-2383429 || 5 | NULL | 白龍馬 | 993267899 |+----+-----------+-----------------+---------------------------------+user2表，悟空的朋友圈+----+--------------+-----------+| id | user_name | comment |+----+--------------+-----------+| 1 | 孫悟空 | 美猴王 || 2 | 牛魔王 | 牛哥 || 3 | 鐵扇公主 | 牛夫人 || 4 | 菩提老祖 | 葡萄 || 5 | NULL | 晶晶 |+----+--------------+-----------+user1_kills表，取經路上殺的妖怪數量+----+-----------+---------------------+-------+| id | user_name | timestr | kills |+----+-----------+---------------------+-------+| 1 | 孫悟空 | 2013-01-10 00:00:00 | 10 || 2 | 孫悟空 | 2013-02-01 00:00:00 | 2 || 3 | 孫悟空 | 2013-02-05 00:00:00 | 12 || 4 | 孫悟空 | 2013-02-12 00:00:00 | 22 || 5 | 豬八戒 | 2013-01-11 00:00:00 | 20 || 6 | 豬八戒 | 2013-02-07 00:00:00 | 17 || 7 | 豬八戒 | 2013-02-08 00:00:00 | 35 || 8 | 沙僧 | 2013-01-10 00:00:00 | 3 || 9 | 沙僧 | 2013-01-22 00:00:00 | 9 || 10 | 沙僧 | 2013-02-11 00:00:00 | 5 |+----+-----------+---------------------+-------+user1_equipment表，取經組裝備+----+-----------+--------------+-----------------+-----------------+| id | user_name | arms | clothing | shoe |+----+-----------+--------------+-----------------+-----------------+| 1 | 唐僧 | 九環錫杖 | 錦斕袈裟 | 僧鞋 || 2 | 孫悟空 | 金箍棒 | 梭子黃金甲 | 藕絲步云履 || 3 | 豬八戒 | 九齒釘耙 | 僧衣 | 僧鞋 || 4 | 沙僧 | 降妖寶杖 | 僧衣 | 僧鞋 |+----+-----------+--------------+-----------------+-----------------+
1. 使用left join優化not in子句
例子：找出取經組中不屬于悟空朋友圈的人
+----+-----------+-----------------+-----------------------+| id | user_name | comment | mobile |+----+-----------+-----------------+-----------------------+| 1 | 唐僧 | 旃檀功德佛 | 138245623,021-382349 || 3 | 豬八戒 | 凈壇使者 | 183208243,055-8234234 || 4 | 沙僧 | 金身羅漢 | 293842295,098-2383429 |+----+-----------+-----------------+-----------------------+
not in寫法：
select * from user1 a where a.user_name not in (select user_name from user2 where user_name is not null);
left join寫法：
首先看通過user_name進行連接的外連接數據集
select a.*, b.* from user1 a left join user2 b on (a.user_name = b.user_name);
+----+-----------+-----------------+---------------------------------+------+-----------+-----------+| id | user_name | comment | mobile | id | user_name | comment |+----+-----------+-----------------+---------------------------------+------+-----------+-----------+| 2 | 孫悟空 | 斗戰勝佛 | 159384292,022-483432,+86-392432 | 1 | 孫悟空 | 美猴王 || 1 | 唐僧 | 旃檀功德佛 | 138245623,021-382349 | NULL | NULL | NULL || 3 | 豬八戒 | 凈壇使者 | 183208243,055-8234234 | NULL | NULL | NULL || 4 | 沙僧 | 金身羅漢 | 293842295,098-2383429 | NULL | NULL | NULL || 5 | NULL | 白龍馬 | 993267899 | NULL | NULL | NULL |+----+-----------+-----------------+---------------------------------+------+-----------+-----------+
可以看到a表中的所有數據都有顯示，b表中的數據只有b.user_name與a.user_name相等才顯示，其余都以null值填充，要想找出取經組中不屬于悟空朋友圈的人，只需要在b.user_name中加一個過濾條件b.user_name is null即可。
select a.* from user1 a left join user2 b on (a.user_name = b.user_name) where b.user_name is null;
+----+-----------+-----------------+-----------------------+| id | user_name | comment | mobile |+----+-----------+-----------------+-----------------------+| 1 | 唐僧 | 旃檀功德佛 | 138245623,021-382349 || 3 | 豬八戒 | 凈壇使者 | 183208243,055-8234234 || 4 | 沙僧 | 金身羅漢 | 293842295,098-2383429 || 5 | NULL | 白龍馬 | 993267899 |+----+-----------+-----------------+-----------------------+
看到這里發現結果集中還多了一個白龍馬，繼續添加過濾條件a.user_name is not null即可。
select a.* from user1 a left join user2 b on (a.user_name = b.user_name) where b.user_name is null and a.user_name is not null;
2. 使用left join優化標量子查詢
例子：查看取經組中的人在悟空朋友圈的昵稱
+-----------+-----------------+-----------+| user_name | comment | comment2 |+-----------+-----------------+-----------+| 唐僧 | 旃檀功德佛 | NULL || 孫悟空 | 斗戰勝佛 | 美猴王 || 豬八戒 | 凈壇使者 | NULL || 沙僧 | 金身羅漢 | NULL || NULL | 白龍馬 | NULL |+-----------+-----------------+-----------+
子查詢寫法：
select a.user_name, a.comment, (select comment from user2 b where b.user_name = a.user_name) comment2 from user1 a;
left join寫法：
select a.user_name, a.comment, b.comment comment2 from user1 a left join user2 b on (a.user_name = b.user_name);
3. 使用join優化聚合子查詢
例子：查詢出取經組中每人打怪最多的日期
+----+-----------+---------------------+-------+| id | user_name | timestr | kills |+----+-----------+---------------------+-------+| 4 | 孫悟空 | 2013-02-12 00:00:00 | 22 || 7 | 豬八戒 | 2013-02-08 00:00:00 | 35 || 9 | 沙僧 | 2013-01-22 00:00:00 | 9 |+----+-----------+---------------------+-------+
聚合子查詢寫法：
select * from user1_kills a where a.kills = (select max(b.kills) from user1_kills b where b.user_name = a.user_name);
join寫法：
首先看兩表自關聯的結果集，為節省篇幅，只取豬八戒的打怪數據來看
select a.*, b.* from user1_kills a join user1_kills b on (a.user_name = b.user_name) order by 1;
+----+-----------+---------------------+-------+----+-----------+---------------------+-------+| id | user_name | timestr | kills | id | user_name | timestr | kills |+----+-----------+---------------------+-------+----+-----------+---------------------+-------+| 5 | 豬八戒 | 2013-01-11 00:00:00 | 20 | 5 | 豬八戒 | 2013-01-11 00:00:00 | 20 || 5 | 豬八戒 | 2013-01-11 00:00:00 | 20 | 6 | 豬八戒 | 2013-02-07 00:00:00 | 17 || 5 | 豬八戒 | 2013-01-11 00:00:00 | 20 | 7 | 豬八戒 | 2013-02-08 00:00:00 | 35 || 6 | 豬八戒 | 2013-02-07 00:00:00 | 17 | 7 | 豬八戒 | 2013-02-08 00:00:00 | 35 || 6 | 豬八戒 | 2013-02-07 00:00:00 | 17 | 5 | 豬八戒 | 2013-01-11 00:00:00 | 20 || 6 | 豬八戒 | 2013-02-07 00:00:00 | 17 | 6 | 豬八戒 | 2013-02-07 00:00:00 | 17 || 7 | 豬八戒 | 2013-02-08 00:00:00 | 35 | 5 | 豬八戒 | 2013-01-11 00:00:00 | 20 || 7 | 豬八戒 | 2013-02-08 00:00:00 | 35 | 6 | 豬八戒 | 2013-02-07 00:00:00 | 17 || 7 | 豬八戒 | 2013-02-08 00:00:00 | 35 | 7 | 豬八戒 | 2013-02-08 00:00:00 | 35 |+----+-----------+---------------------+-------+----+-----------+---------------------+-------+
可以看到當兩表通過user_name進行自關聯，只需要對a表的所有字段進行一個group by，取b表中的max(kills)，只要a.kills=max(b.kills)就滿足要求了。sql如下
select a.* from user1_kills a join user1_kills b on (a.user_name = b.user_name) group by a.id, a.user_name, a.timestr, a.kills having a.kills = max(b.kills);
4. 使用join進行分組選擇
例子：對第3個例子進行升級，查詢出取經組中每人打怪最多的前兩個日期
+----+-----------+---------------------+-------+| id | user_name | timestr | kills |+----+-----------+---------------------+-------+| 3 | 孫悟空 | 2013-02-05 00:00:00 | 12 || 4 | 孫悟空 | 2013-02-12 00:00:00 | 22 || 5 | 豬八戒 | 2013-01-11 00:00:00 | 20 || 7 | 豬八戒 | 2013-02-08 00:00:00 | 35 || 9 | 沙僧 | 2013-01-22 00:00:00 | 9 || 10 | 沙僧 | 2013-02-11 00:00:00 | 5 |+----+-----------+---------------------+-------+
在oracle中，可以通過分析函數來實現
select b.* from (select a.*, row_number() over(partition by user_name order by kills desc) cnt from user1_kills a) b where b.cnt <= 2;
很遺憾，上面sql在mysql中報錯ERROR 1064 (42000): You have an error in your SQL syntax; 因為mysql并不支持分析函數。不過可以通過下面的方式去實現。
首先對兩表進行自關聯，為了節約篇幅，只取出孫悟空的數據
select a.*, b.* from user1_kills a join user1_kills b on (a.user_name=b.user_name and a.kills<=b.kills) order by a.user_name, a.kills desc;
+----+-----------+---------------------+-------+----+-----------+---------------------+-------+| id | user_name | timestr | kills | id | user_name | timestr | kills |+----+-----------+---------------------+-------+----+-----------+---------------------+-------+| 4 | 孫悟空 | 2013-02-12 00:00:00 | 22 | 4 | 孫悟空 | 2013-02-12 00:00:00 | 22 || 3 | 孫悟空 | 2013-02-05 00:00:00 | 12 | 3 | 孫悟空 | 2013-02-05 00:00:00 | 12 || 3 | 孫悟空 | 2013-02-05 00:00:00 | 12 | 4 | 孫悟空 | 2013-02-12 00:00:00 | 22 || 1 | 孫悟空 | 2013-01-10 00:00:00 | 10 | 1 | 孫悟空 | 2013-01-10 00:00:00 | 10 || 1 | 孫悟空 | 2013-01-10 00:00:00 | 10 | 3 | 孫悟空 | 2013-02-05 00:00:00 | 12 || 1 | 孫悟空 | 2013-01-10 00:00:00 | 10 | 4 | 孫悟空 | 2013-02-12 00:00:00 | 22 || 2 | 孫悟空 | 2013-02-01 00:00:00 | 2 | 1 | 孫悟空 | 2013-01-10 00:00:00 | 10 || 2 | 孫悟空 | 2013-02-01 00:00:00 | 2 | 3 | 孫悟空 | 2013-02-05 00:00:00 | 12 || 2 | 孫悟空 | 2013-02-01 00:00:00 | 2 | 4 | 孫悟空 | 2013-02-12 00:00:00 | 22 || 2 | 孫悟空 | 2013-02-01 00:00:00 | 2 | 2 | 孫悟空 | 2013-02-01 00:00:00 | 2 |+----+-----------+---------------------+-------+----+-----------+---------------------+-------+
從上面的表中我們知道孫悟空打怪前兩名的數量是22和12，那么只需要對a表的所有字段進行一個group by，對b表的id做個count，count值小于等于2就滿足要求，sql改寫如下：
select a.* from user1_kills a join user1_kills b on (a.user_name=b.user_name and a.kills<=b.kills) group by a.id, a.user_name, a.timestr, a.kills having count(b.id) <= 2;
5. 使用笛卡爾積關聯實現一列轉多行
例子：將取經組中每個電話號碼變成一行
原始數據：
+-----------+---------------------------------+| user_name | mobile |+-----------+---------------------------------+| 唐僧 | 138245623,021-382349 || 孫悟空 | 159384292,022-483432,+86-392432 || 豬八戒 | 183208243,055-8234234 || 沙僧 | 293842295,098-2383429 || NULL | 993267899 |+-----------+---------------------------------+
想要得到的數據：
+-----------+-------------+| user_name | mobile |+-----------+-------------+| 唐僧 | 138245623 || 唐僧 | 021-382349 || 孫悟空 | 159384292 || 孫悟空 | 022-483432 || 孫悟空 | +86-392432 || 豬八戒 | 183208243 || 豬八戒 | 055-8234234 || 沙僧 | 293842295 || 沙僧 | 098-2383429 || NULL | 993267899 |+-----------+-------------+
可以看到唐僧有兩個電話，因此他就需要兩行。我們可以先求出每人的電話號碼數量，然后與一張序列表進行笛卡兒積關聯，為了節約篇幅，只取出唐僧的數據
select a.id, b.* from tb_sequence a cross join (select user_name, mobile, length(mobile)-length(replace(mobile, ',', ''))+1 size from user1) b order by 2,1;
+----+-----------+---------------------------------+------+| id | user_name | mobile | size |+----+-----------+---------------------------------+------+| 1 | 唐僧 | 138245623,021-382349 | 2 || 2 | 唐僧 | 138245623,021-382349 | 2 || 3 | 唐僧 | 138245623,021-382349 | 2 || 4 | 唐僧 | 138245623,021-382349 | 2 || 5 | 唐僧 | 138245623,021-382349 | 2 || 6 | 唐僧 | 138245623,021-382349 | 2 || 7 | 唐僧 | 138245623,021-382349 | 2 || 8 | 唐僧 | 138245623,021-382349 | 2 || 9 | 唐僧 | 138245623,021-382349 | 2 || 10 | 唐僧 | 138245623,021-382349 | 2 |+----+-----------+---------------------------------+------+
a.id對應的就是第幾個電話號碼，size就是總的電話號碼數量，因此可以加上關聯條件(a.id <= b.size)，將上面的sql繼續調整
select b.user_name, replace(substring(substring_index(b.mobile, ',', a.id), char_length(substring_index(mobile, ',', a.id-1)) + 1), ',', '') as mobile from tb_sequence a cross join (select user_name, concat(mobile, ',') as mobile, length(mobile)-length(replace(mobile, ',', ''))+1 size from user1) b on (a.id <= b.size);
6. 使用笛卡爾積關聯實現多列轉多行
例子：將取經組中每件裝備變成一行
原始數據：
+----+-----------+--------------+-----------------+-----------------+| id | user_name | arms | clothing | shoe |+----+-----------+--------------+-----------------+-----------------+| 1 | 唐僧 | 九環錫杖 | 錦斕袈裟 | 僧鞋 || 2 | 孫悟空 | 金箍棒 | 梭子黃金甲 | 藕絲步云履 || 3 | 豬八戒 | 九齒釘耙 | 僧衣 | 僧鞋 || 4 | 沙僧 | 降妖寶杖 | 僧衣 | 僧鞋 |+----+-----------+--------------+-----------------+-----------------+
想要得到的數據：
+-----------+-----------+-----------------+| user_name | equipment | equip_mame |+-----------+-----------+-----------------+| 唐僧 | arms | 九環錫杖 || 唐僧 | clothing | 錦斕袈裟 || 唐僧 | shoe | 僧鞋 || 孫悟空 | arms | 金箍棒 || 孫悟空 | clothing | 梭子黃金甲 || 孫悟空 | shoe | 藕絲步云履 || 沙僧 | arms | 降妖寶杖 || 沙僧 | clothing | 僧衣 || 沙僧 | shoe | 僧鞋 || 豬八戒 | arms | 九齒釘耙 || 豬八戒 | clothing | 僧衣 || 豬八戒 | shoe | 僧鞋 |+-----------+-----------+-----------------+
union的寫法：
select user_name, 'arms' as equipment, arms equip_mame from user1_equipmentunion allselect user_name, 'clothing' as equipment, clothing equip_mame from user1_equipmentunion allselect user_name, 'shoe' as equipment, shoe equip_mame from user1_equipmentorder by 1, 2;
join的寫法：
首先看笛卡爾數據集的效果，以唐僧為例
select a.*, b.* from user1_equipment a cross join tb_sequence b where b.id <= 3;
+----+-----------+--------------+-----------------+-----------------+----+| id | user_name | arms | clothing | shoe | id |+----+-----------+--------------+-----------------+-----------------+----+| 1 | 唐僧 | 九環錫杖 | 錦斕袈裟 | 僧鞋 | 1 || 1 | 唐僧 | 九環錫杖 | 錦斕袈裟 | 僧鞋 | 2 || 1 | 唐僧 | 九環錫杖 | 錦斕袈裟 | 僧鞋 | 3 |+----+-----------+--------------+-----------------+-----------------+----+
使用case對上面的結果進行處理
select user_name, case when b.id = 1 then 'arms' when b.id = 2 then 'clothing'when b.id = 3 then 'shoe' end as equipment,case when b.id = 1 then arms end arms,case when b.id = 2 then clothing end clothing,case when b.id = 3 then shoe end shoefrom user1_equipment a cross join tb_sequence b where b.id <=3;
+-----------+-----------+--------------+-----------------+-----------------+| user_name | equipment | arms | clothing | shoe |+-----------+-----------+--------------+-----------------+-----------------+| 唐僧 | arms | 九環錫杖 | NULL | NULL || 唐僧 | clothing | NULL | 錦斕袈裟 | NULL || 唐僧 | shoe | NULL | NULL | 僧鞋 |+-----------+-----------+--------------+-----------------+-----------------+
使用coalesce函數將多列數據進行合并
select user_name, case when b.id = 1 then 'arms' when b.id = 2 then 'clothing'when b.id = 3 then 'shoe' end as equipment,coalesce(case when b.id = 1 then arms end,case when b.id = 2 then clothing end,case when b.id = 3 then shoe end) equip_mamefrom user1_equipment a cross join tb_sequence b where b.id <=3 order by 1, 2;
7. 使用join更新過濾條件中包含自身的表
例子：把同時存在于取經組和悟空朋友圈中的人，在取經組中把comment字段更新為"此人在悟空的朋友圈"
我們很自然地想到先查出user1和user2中user_name都存在的人，然后更新user1表，sql如下
update user1 set comment = '此人在悟空的朋友圈' where user_name in (select a.user_name from user1 a join user2 b on (a.user_name = b.user_name));
很遺憾，上面sql在mysql中報錯：ERROR 1093 (HY000): You can't specify target table 'user1' for update in FROM clause，提示不能更新目標表在from子句的表。
那有沒有其它辦法呢？我們可以將in的寫法轉換成join的方式
select c.*, d.* from user1 c join (select a.user_name from user1 a join user2 b on (a.user_name = b.user_name)) d on (c.user_name = d.user_name);
+----+-----------+--------------+---------------------------------+-----------+| id | user_name | comment | mobile | user_name |+----+-----------+--------------+---------------------------------+-----------+| 2 | 孫悟空 | 斗戰勝佛 | 159384292,022-483432,+86-392432 | 孫悟空 |+----+-----------+--------------+---------------------------------+-----------+
然后對join之后的視圖進行更新即可
update user1 c join (select a.user_name from user1 a join user2 b on (a.user_name = b.user_name)) d on (c.user_name = d.user_name) set c.comment = '此人在悟空的朋友圈';
再查看user1，可以看到user1已修改成功
select * from user1;
+----+-----------+-----------------------------+---------------------------------+| id | user_name | comment | mobile |+----+-----------+-----------------------------+---------------------------------+| 1 | 唐僧 | 旃檀功德佛 | 138245623,021-382349 || 2 | 孫悟空 | 此人在悟空的朋友圈 | 159384292,022-483432,+86-392432 || 3 | 豬八戒 | 凈壇使者 | 183208243,055-8234234 || 4 | 沙僧 | 金身羅漢 | 293842295,098-2383429 || 5 | NULL | 白龍馬 | 993267899 |+----+-----------+-----------------------------+---------------------------------+
8. 使用join刪除重復數據
首先向user2表中插入兩條數據
insert into user2(user_name, comment) values ('孫悟空', '美猴王');insert into user2(user_name, comment) values ('牛魔王', '牛哥');例子：將user2表中的重復數據刪除，只保留id號大的
+----+--------------+-----------+| id | user_name | comment |+----+--------------+-----------+| 1 | 孫悟空 | 美猴王 || 2 | 牛魔王 | 牛哥 || 3 | 鐵扇公主 | 牛夫人 || 4 | 菩提老祖 | 葡萄 || 5 | NULL | 晶晶 || 6 | 孫悟空 | 美猴王 || 7 | 牛魔王 | 牛哥 |+----+--------------+-----------+
首先查看重復記錄
select a.*, b.* from user2 a join (select user_name, comment, max(id) id from user2 group by user_name, comment having count(*) > 1) b on (a.user_name=b.user_name and a.comment=b.comment) order by 2;
+----+-----------+-----------+-----------+-----------+------+| id | user_name | comment | user_name | comment | id |+----+-----------+-----------+-----------+-----------+------+| 1 | 孫悟空 | 美猴王 | 孫悟空 | 美猴王 | 6 || 6 | 孫悟空 | 美猴王 | 孫悟空 | 美猴王 | 6 || 2 | 牛魔王 | 牛哥 | 牛魔王 | 牛哥 | 7 || 7 | 牛魔王 | 牛哥 | 牛魔王 | 牛哥 | 7 |+----+-----------+-----------+-----------+-----------+------+
接著只需要刪除(a.id < b.id)的數據即可
delete a from user2 a join (select user_name, comment, max(id) id from user2 group by user_name, comment having count(*) > 1) b on (a.user_name=b.user_name and a.comment=b.comment) where a.id < b.id;
查看user2，可以看到重復數據已經被刪掉了
select * from user2;
+----+--------------+-----------+| id | user_name | comment |+----+--------------+-----------+| 3 | 鐵扇公主 | 牛夫人 || 4 | 菩提老祖 | 葡萄 || 5 | NULL | 晶晶 || 6 | 孫悟空 | 美猴王 || 7 | 牛魔王 | 牛哥 |+----+--------------+-----------+
總結：
給大家就介紹到這里，大家有興趣可以多造點數據，然后比較不同的sql寫法在執行時間上的區別。本文例子取自于慕課網《sql開發技巧》。
好了，以上就是這篇文章的全部內容了，希望本文的內容對大家的學習或者工作具有一定的參考學習價值，謝謝大家對武林網的支持。
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