一、如何进行行列或者列行的转换
行转列的关键是union ,列转行的关键是join
需要进行行转列的场景
报表的设计
(假设A表是一行一行的数据 分别是日期对应的销售金额有多行。需要将其转换为一列一列的数据 第一行是日期 第二行是销售金额)
汇总的显示 需求分析
将A表结构转至成B 再将B结构转至成C
mysql> select a.user_name as 姓名,c.kills as 打怪数 from a join c on a.id = c.user_id;
A +-----------+-----------+
| 姓名 | 打怪数 |
+-----------+-----------+
| 猪八戒 | 10 |
| 猪八戒 | 2 |
| 猪八戒 | 12 |
| 沙僧 | 3 |
| 沙僧 | 5 |
| 孙悟空 | 1 |
| 孙悟空 | 20 |
| 唐僧 | 10 |
| 唐僧 | 17 |
+-----------+-----------+
9 rows in set (0.00 sec)
mysql> select a.user_name,sum(c.kills) from a join c on a.id = c.user_id group by a.user_name;
B +-----------+--------------+
| user_name | sum(c.kills) |
+-----------+--------------+
| 唐僧 | 27 |
| 孙悟空 | 21 |
| 沙僧 | 8 |
| 猪八戒 | 24 |
+-----------+--------------+
4 rows in set (0.00 sec)
结果1 使用Cross Join—————交叉连接(类笛卡尔积的方法)
mysql>
SELECT
*
FROM
(
SELECT
sum( c.kills ) AS '孙悟空'
FROM
a
JOIN c ON a.id = c.user_id
AND a.user_name = '孙悟空'
GROUP BY
a.user_name
) a
CROSS JOIN (
SELECT
sum( c.kills ) AS '猪八戒'
FROM
a
JOIN c ON a.id = c.user_id
AND a.user_name = '猪八戒'
GROUP BY
a.user_name
) b
CROSS JOIN (
SELECT
sum( c.kills ) AS '沙僧'
FROM
a
JOIN c ON a.id = c.user_id
AND a.user_name = '沙僧'
GROUP BY
a.user_name
) c;
+-----------+-----------+--------+
| 孙悟空 | 猪八戒 | 沙僧 |
+-----------+-----------+--------+
| 21 | 24 | 8 |
+-----------+-----------+--------+
1 row in set (0.00 sec)
结果2 使用case case when 条件 then 获取结果 end
mysql>
SELECT
sum(
CASE
WHEN a.user_name = '猪八戒' THEN
c.kills
END
) AS 猪八戒,
sum(
CASE
WHEN a.user_name = '沙僧' THEN
c.kills
END
) AS 沙僧,
sum(
CASE
WHEN a.user_name = '孙悟空' THEN
c.kills
END
) AS 孙悟空
FROM
a
JOIN c ON a.id = c.user_id;
C +-----------+--------+-----------+
| 猪八戒 | 沙僧 | 孙悟空 |
+-----------+--------+-----------+
| 24 | 8 | 21 |
+-----------+--------+-----------+
1 row in set (0.00 sec)
单列转多行场景介绍
mysql>
SELECT
a.user_name,
REPLACE (
substring(
substring_index( mobile, ',', a.id ),
char_length(
substring_index( mobile, ',', a.id - 1 )
) + 1
),
',',
''
) AS mobile
FROM
tb_sequence a
CROSS JOIN (
SELECT
a.user_name,
concat( mobile, ',' ) AS mobile,
length( mobile ) - length( REPLACE ( mobile, ',', '' ) ) + 1 size
FROM
a
) a ON a.id <= b.size;
mysql>
SELECT
a.user_name,
concat( over, ',' ) AS mobile,
length( over ) - length( REPLACE ( over, ',', '' ) ) + 1 size
FROM
a;
+-----------+--------------------------------------------+------+
| user_name | mobile | size |
+-----------+--------------------------------------------+------+
| 唐僧 | 213312,31231231,32121312321, | 3 |
| 猪八戒 | 321324213,3214213, | 2 |
| 孙悟空 | 534223423,423523321,3242342342,432423423, | 4 |
| 沙僧 | 543123124,654654543, | 2 |
+-----------+--------------------------------------------+------+
4 rows in set (0.00 sec)
SQL: coalesce()函数
①用途：
将空值替换成其他值
返回第一个非空值
②表达式：
COALESCE是一个函数， (expression_1, expression_2, ...,expression_n)依次参考各参数表达式，遇到非null值即停止并返回该值。如果所有的表达式都是空值，最终将返回一个空值。使用COALESCE在于大部分包含空值的表达式最终将返回空值。
coalesce(item1,item2,item3)
依次判断item值，返回第一个不为null的值，若全为null，则返回null；
③实例：
mysql> select a.id,a.user_name,a.over,coalesce(b.id,'无') as id,coalesce(b.user_name,'佚名') as 姓名, coalesce(b.over,'无业') as 职业
from a
left join b on a.user_name = b.user_name;
+----+-----------+------------------------------------+------+-----------+--------------+
| id | user_name | over | id | 姓名 | 职业 |
+----+-----------+------------------------------------+------+-----------+--------------+
| 3 | 孙悟空 | 534223423,423523321,324234234.. | 1 | 孙悟空 | 齐天大圣 |
| 1 | 唐僧 | 213312,31231231,32121312321, | 无 | 佚名 | 无业 |
| 2 | 猪八戒 | 321324213,3214213, | 无 | 佚名 | 无业 |
| 4 | 沙僧 | 543123124,654654543, | 无 | 佚名 | 无业 |
+----+-----------+------------------------------------+------+-----------+--------------+
4 rows in set (0.00 sec)二、如何生成唯一序列号（常见于票据的生成或者主键的生成）
需要使用唯一序列号的场景
1.数据库的主键
主键选择时 尽可能的要小、唯一不重复
2.业务序列号
发票、车票、订单
序列号的生成
1. MySQL中 : AUTO_INCERMENT
2. SQLServer: IDENTITY/SEQUENCE
3. Orale : SEQUENCE
4. PgSql : SEQUENCE 如何选择生成序列号的方式
1. 优先选择系统提供的序列号选择方式 (缺点是：容易产生空洞、在事物操作插入之后回滚 会占用序列号)
2. 在特殊情况下可以使用SQL方式生成序列号
如何使用SQL语句建立特殊需求的序列号
需求：生成订单的序列号，并且订单格式如下
YYYYMMDDHHMMSSS。如201807200000002
DECLARE v_cnt INT;
DECLARE v_timestr INT;
DECLARE v_timestr = DATE_FORMAT(NOW(),'%Y%m%d');
SELECT ROUND(RAND()*100,0)+1 INTO v_cnt;
START TRANSACTION;
UPDATE order_seq(timestr,order_sn)+v_cnt WHERE timestr = v_timestr;
IF ROW_COUNT() = 0 THEN
INSERT INTO order_seq(timestr,order_sn) VALUES(v_timestr,v_cnt);
END IF;
SELECT CONCAT(v_timestr,LPAD(order_sn,7,0)) AS order_sn
FROM order_seq WHERE timestr = v_timestr;
COMMIT; UPDATE order_seq SET order_sn = order_sn + v_cnt WHERE timestr = v_timestr;
IF ROW_COUNT() = 0 THEN
INSERT INTO order_seq(timestr,order_sn) VALUES(v_timestr,v_cnt);
END IF;
SELECT CONCAT(v_timestr,LPAD(order_sn,7,0))AS order_sn
FROM order_seq WHERE timestr = v_timestr;
COMMIT;三、如何删除重复数据
产生数据重复的原因
1. 人为原因、如重复录入数据、重复提交 等
2. 系统原因、由于系统升级或设计的原因使原来可以重复的数据变为不能重复了。
如何查询数据是否重复?
利用group by和having从句处理
sql数据格式显示
mysql> select * from a;
+----+-----------+-------------------------------------------+
| id | user_name | over |
+----+-----------+-------------------------------------------+
| 1 | 唐僧 | 213312,31231231,32121312321, |
| 2 | 猪八戒 | 321324213,3214213, |
| 3 | 孙悟空 | 534223423,423523321,3242342342,432423423, |
| 4 | 沙僧 | 543123124,654654543, |
| 5 | 唐僧 | 321321313，321312 |
| 6 | 唐僧 | 321313,32321 |
+----+-----------+-------------------------------------------+
6 rows in set (0.00 sec)
1. 处理方式
采用Group by 与 Having 查看数据是否重复
Having解释： 如果需要对组函数的结果作为条件，那么不能使用where子句，必须使用having子句。
写法1.(查看重复数据的数量)
mysql> select user_name,count(*) from a group by user_name having count(*) > 1;
+-----------+----------+
| user_name | count(*) |
+-----------+----------+
| 唐僧 | 3 |
+-----------+----------+
1 row in set (0.00 sec) 2. 删除简单重复数据，对于相同数据保留ID最大的
第一步：
mysql> select user_name,count(*) as 出现次数,max(id) as id from a GROUP BY user_name having count(*) > 1 ;
+-----------+--------------+------+
| user_name | 出现次数 | id |
+-----------+--------------+------+
| 唐僧 | 3 | 6 |
| 孙悟空 | 2 | 7 |
+-----------+--------------+------+
2 rows in set (0.00 sec)
第二步：
mysql> delete a from a join(select user_name,count(*),max(id) as id from a GROUP BY user_name having count(*) > 1) b on a.user_name = b.user_name where a.id < b.id;
Query OK, 3 rows affected (0.00 sec) mysql> select * from a;
+----+-----------+-----------------+
| id | user_name | over |
+----+-----------+-----------------+
| 6 | 唐僧 | 321313,32321 |
| 7 | 孙悟空 | 321,321321 |
| 9 | 沙僧 | 423434,32432432 |
| 10 | 猪八戒 | 24324,43243223 |
+----+-----------+-----------------+
4 rows in set (0.00 sec) 3. 删除复杂重复数据
mysql> select * from a;
重复的数据在 over列中的321
+----+-----------+----------------+
| id | user_name | over |
+----+-----------+----------------+
| 6 | 唐僧 | 321,32321 |
| 7 | 孙悟空 | 321,321321 |
| 9 | 沙僧 | 423434,321,321 |
| 10 | 猪八戒 | 24324,43243223 |
+----+-----------+----------------+
4 rows in set (0.00 sec) 删除某个列中的重复数据
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