MYSQL用法(十四) MySQL中字符串连接函数

一 concat 函数

使用方法： CONCAT(str1,str2,…)

返回结果为连接参数产生的字符串。如有任何一个参数为NULL ，则返回值为 NULL。

注意：

如果所有参数均为非二进制字符串，则结果为非二进制字符串。

如果自变量中含有任一二进制字符串，则结果为一个二进制字符串。

一个数字参数被转化为与之相等的二进制字符串格式；若要避免这种情况，可使用显式类型 cast, 例如：

>SELECT CONCAT(CAST(int_col AS CHAR), char_col)

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)

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)

二 concat_ws 函数

使用方法： CONCAT_WS(separator,str1,str2,...) CONCAT_WS() 代表 CONCAT With Separator ，是CONCAT()的特殊形式。第一个参数是其它参数的分隔符。
分隔符的位置放在要连接的两个字符串之间。分隔符可以是一个字符串，也可以是其它参数。

注意：

如果分隔符为 NULL，则结果为 NULL。函数会忽略任何分隔符参数后的 NULL 值。

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MySQL中concat函数不同的是, 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)

三 group_concat 函数

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'])

基本查询

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)

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)

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)

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)

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)

四 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)

mysql向表中某字段后追加一段字符串:

update table_name set field=CONCAT(field,'',str)

mysql 向表中某字段前加字符串

update table_name set field=CONCAT('str',field)

实例:

SELECT
staff.ID AS id,
staff.STAFF_ID AS staffId,
staff.STAFF_NAME AS staffName,
staff.STAFF_LOGIN_NAME AS staffLoginName,
staff.STAFF_CODE AS staffCode,
staff.STAFF_STATUS AS staffStatus,
staff.STAFF_MOBILE AS staffMobile,
staff.STAFF_PHONE AS staffPhone,
staff.STAFF_EMAIL AS staffEmail,
staff.STAFF_QQ AS staffQq,
staff.STAFF_REMARK AS staffRemark,
staff.FREEZE_STATUS AS freezeStatus,
staff.FREEZE_REMARK AS freezeRemark,
staff.STAFF_PIC_ATTACHID AS staffPicAttachid,
staff.STAFF_PIC_URL AS staffPicUrl,
staff.STAFF_DEPART_CODE AS staffDepartCode,
staff.STAFF_DEPART_NAME AS staffDepartName,
staff.LAST_LOGIN_TIME AS lastLoginTime,
staff.CREATE_GMT AS createGmt,
staff.CREATE_USERID AS createUserid,
staff.CREATE_USERNAME AS createUsername,
staff.UPDATE_GMT AS updateGmt,
staff.UPDATE_USERID AS updateUserid,
staff.UPDATE_USERNAME AS updateUsername,
group_concat(rel.ROLE_CODE,"") AS roleCode
FROM
SYS_STAFF AS staff
LEFT JOIN sys_staff_role_rel AS rel
ON rel.STAFF_ID = staff.STAFF_ID
WHERE
staff.STAFF_ID='0cf01ea9b8214714a970149cd35f3137'
GROUP BY  staff.STAFF_ID
ORDER BY
staff.ID ASC

结果显示: roleCode 这行将去重合并显示