1、concat
()函数
1.1 MySQL的concat函数可以连接一个或者多个字符串,如
mysql> select concat(‘10‘);
+--------------+
| concat(‘10‘) |
+--------------+
| 10 |
+--------------+
1 row in set (0.00 sec)
mysql> select concat(‘11‘,‘22‘,‘33‘);
+------------------------+
| concat(‘11‘,‘22‘,‘33‘) |
+------------------------+
| 112233 |
+------------------------+
1 row in set (0.00 sec)
而Oracle的concat函数只能连接两个字符串
SQL> select concat(‘11‘,‘22‘) from dual;
1.2 MySQL的concat函数在连接字符串的时候，只要其中一个是NULL,那么将返回NULL
mysql> select concat(‘11‘,‘22‘,null);
+------------------------+
| concat(‘11‘,‘22‘,null) |
+------------------------+
| NULL |
+------------------------+
1 row in set (0.00 sec)
而Oracle的concat函数连接的时候，只要有一个字符串不是NULL,就不会返回NULL
SQL> select concat(‘11‘,NULL) from dual;
CONCAT
--
11
2、concat_ws()函数, 表示concat with separator,即有分隔符的字符串连接
如连接后以逗号分隔
mysql> select concat_ws(‘,‘,‘11‘,‘22‘,‘33‘);
+-------------------------------+
| concat_ws(‘,‘,‘11‘,‘22‘,‘33‘) |
+-------------------------------+
| 11,22,33 |
+-------------------------------+
1 row in set (0.00 sec)
和concat不同的是, concat_ws函数在执行的时候,不会因为NULL值而返回NULL
mysql> select concat_ws(‘,‘,‘11‘,‘22‘,NULL);
+-------------------------------+
| concat_ws(‘,‘,‘11‘,‘22‘,NULL) |
+-------------------------------+
| 11,22 |
+-------------------------------+
1 row in set (0.00 sec)
3、group_concat()可用来行转列, Oracle没有这样的函数
完整的语法如下
group_concat([DISTINCT] 要连接的字段 [Order BY ASC/DESC 排序字段] [Separator ‘分隔符‘])
如下例子
mysql> select * from aa;
+------+------+
| id | name |
+------+------+
| 1 | 10 |
| 1 | 20 |
| 1 | 20 |
| 2 | 20 |
| 3 | 200 |
| 3 | 500 |
+------+------+
6 rows in set (0.00 sec)
3.1 以id分组，把name字段的值打印在一行，逗号分隔(默认)
mysql> select id,group_concat(name) from aa group by id;
+------+--------------------+
| id | group_concat(name) |
+------+--------------------+
| 1 | 10,20,20 |
| 2 | 20 |
| 3 | 200,500 |
+------+--------------------+
3 rows in set (0.00 sec)
3.2 以id分组，把name字段的值打印在一行，分号分隔
mysql> select id,group_concat(name separator ‘;‘) from aa group by id;
+------+----------------------------------+
| id | group_concat(name separator ‘;‘) |
+------+----------------------------------+
| 1 | 10;20;20 |
| 2 | 20 |
| 3 | 200;500 |
+------+----------------------------------+
3 rows in set (0.00 sec)
3.3 以id分组，把去冗余的name字段的值打印在一行，逗号分隔
mysql> select id,group_concat(distinct name) from aa group by id;
+------+-----------------------------+
| id | group_concat(distinct name) |
+------+-----------------------------+
| 1 | 10,20 |
| 2 | 20 |
| 3 | 200,500 |
+------+-----------------------------+
3 rows in set (0.00 sec)
3.4 以id分组，把name字段的值打印在一行，逗号分隔,以name排倒序
mysql> select id,group_concat(name order by name desc) from aa group by id;
+------+---------------------------------------+
| id | group_concat(name order by name desc) |
+------+---------------------------------------+
| 1 | 20,20,10 |
| 2 | 20 |
| 3 | 500,200 |
+------+---------------------------------------+
3 rows in set (0.00 sec)
4、repeat()函数，用来复制字符串,如下‘ab‘表示要复制的字符串，2表示复制的份数
mysql> select repeat(‘ab‘,2);
+----------------+
| repeat(‘ab‘,2) |
+----------------+
| abab |
+----------------+
1 row in set (0.00 sec)
又如
mysql> select repeat(‘a‘,2);
+---------------+
| repeat(‘a‘,2) |
+---------------+
| aa |
+---------------+
1 row in set (0.00 sec)
5、SPACE(N) 函数。生成N个空格，如
mysql> select space(3);
+----------+
| space(3) |
+----------+
| |
+----------+
1 row in set (0.00 sec)
6、STRCMP(STR1,STR2) 字符串比较函数,该函数和字符集有关系，默认区分大小写
若STR1和STR2相同, 则返回 0,
若STR1小于STR2, 则返回 -1,
若STR1大于STR2, 则返回 1
mysql> select strcmp(‘abc‘,‘abc‘);
+---------------------+
| strcmp(‘abc‘,‘abc‘) |
+---------------------+
| 0 |
+---------------------+
1 row in set (0.00 sec)
mysql> select strcmp(‘a‘,‘ab‘);
+------------------+
| strcmp(‘a‘,‘ab‘) |
+------------------+
| -1 |
+------------------+
1 row in set (0.00 sec)
mysql> select strcmp(‘abc‘,‘ab‘);
+--------------------+
| strcmp(‘abc‘,‘ab‘) |
+--------------------+
| 1 |
+--------------------+
1 row in set (0.00 sec)