mysql字符函数

1、CONCAT()  字符连接

（1）mysql> SELECT CONCAT('imooc', 'MYSQL');
+--------------------------+
| CONCAT('imooc', 'MYSQL') |
+--------------------------+
| imoocMYSQL                   |
+--------------------------+

（2）mysql> SELECT CONCAT('imooc','-','MYSQL');
+-----------------------------+
| CONCAT('imooc','-','MYSQL') |
+-----------------------------+
| imooc-MYSQL                     |
+-----------------------------+

（3）mysql> SELECT * FROM test;
+----+----------+
| id | username |
+----+----------+
| 1 | John |
| 2 | Mary |
+----+----------+
2 rows in set (0.00 sec)

mysql> SELECT CONCAT(id, username) AS id_username FROM test;
+-------------+
| id_username |
+-------------+
| 1John |
| 2Mary |
+-------------+

2、CONCAT_WS()   使用指定的分隔符进行字符连接（最少三个字符）

mysql> SELECT CONCAT_WS('|', 'A','B','C');
+-----------------------------+
| CONCAT_WS('|', 'A','B','C') |
+-----------------------------+
| A|B|C |
+-----------------------------+
1 row in set (0.00 sec)

mysql>
mysql> SELECT CONCAT_WS('-','imooc','MYSQL');
+--------------------------------+
| CONCAT_WS('-','imooc','MYSQL') |
+--------------------------------+
| imooc-MYSQL |
+--------------------------------+

3、FORMAT()  数字格式化

mysql> ) |
+--------------------+
| 12,560. |
+--------------------+
1 row in set (0.02 sec)

mysql>
mysql> SELECT FORMAT(12560.72, |
+--------------------+
1 row in set (0.00 sec)

mysql> SELECT FORMAT(12560.72,);
+--------------------+
| FORMAT(12560.72,0) |
+--------------------+
| 12,561                   |

4、LOWER()   UPPER()

mysql> SELECT LOWER('Mysql');
+----------------+
| LOWER('Mysql') |
+----------------+
| mysql              |
+----------------+
1 row in set (0.00 sec)

mysql> SELECT UPPER('Mysql');
+----------------+
| UPPER('Mysql') |
+----------------+
| MYSQL            |
+----------------+
1 row in set (0.00 sec)

5、LEFT()   获取左侧字符

     RIGHT()  获取右侧字符

mysql> SELECT LEFT('MySQL', 2);
+------------------+
| LEFT('MySQL', 2) |
+------------------+
| My |
+------------------+
1 row in set (0.00 sec)

mysql> SELECT LOWER( LEFT('MySQL', 2) );
+---------------------------+
| LOWER( LEFT('MySQL', 2) ) |
+---------------------------+
| my |
+---------------------------+

6、LENGTH()  获取字符串长度

mysql> SELECT LENGTH('MYSQL');
+-----------------+
| LENGTH('MYSQL') |
+-----------------+
| 5       |
+-----------------+
1 row in set (0.01 sec)

mysql> SELECT LENGTH('MYSQ L');
+------------------+
| LENGTH('MYSQ L') |
+------------------+
| 6     |
+------------------+

7、LTRIM()  删除前导空格（第一个字符之前的）

    RTRIM()   删除后续空格（最后一个空格之后的）

    TRIM()   删除前导和后续空格

mysql> SELECT LENGTH('  MYSQL    ');
+-----------------------+
| LENGTH(' MYSQL ') |
+-----------------------+
| 11 |
+-----------------------+

mysql> SELECT LENGTH( |
+------------------------------+
1 row in set (0.00 sec)

mysql> SELECT LENGTH( |
+------------------------------+
1 row in set (0.00 sec)

mysql> SELECT LENGTH( |
+-----------------------------+
1 row in set (0.02 sec)

mysql> SELECT TRIM(LEADING '?' FROM '??MySQL???');  #删除前导字符？
+-------------------------------------+
| TRIM(LEADING '?' FROM '??MySQL???') |
+-------------------------------------+
| MySQL??? |
+-------------------------------------+

mysql> SELECT TRIM(TRAILING '?' FROM '??MySQL???');  #删除后续字符？
+--------------------------------------+
| TRIM(TRAILING '?' FROM '??MySQL???') |
+--------------------------------------+
| ??MySQL |
+--------------------------------------+
1 row in set (0.00 sec)

mysql> SELECT TRIM(BOTH '?' FROM '??MySQL???');  #删除前导和后续字符？
+----------------------------------+
| TRIM(BOTH '?' FROM '??MySQL???') |
+----------------------------------+
| MySQL |
+----------------------------------+
1 row in set (0.00 sec)

8、REPLACE()  

mysql> SELECT TRIM(BOTH '?' FROM '??My??SQL???');
+------------------------------------+
| TRIM(BOTH '?' FROM '??My??SQL???') |
+------------------------------------+
| My??SQL |
+------------------------------------+
1 row in set (0.00 sec)

mysql> SELECT REPLACE('??My??SQL???', '?', '');  #将字符串中的？替换成空格
+----------------------------------+
| REPLACE('??My??SQL???', '?', '') |
+----------------------------------+
| MySQL |
+----------------------------------+
1 row in set (0.01 sec)

9、SUBSTRING()  字符串的截取

mysql> SELECT SUBSTRING('MySQL', 1, 2);   #从字符串第一位开始截取，截取两个
+--------------------------+ 
| SUBSTRING('MySQL', 1, 2) |
+--------------------------+
| My |
+--------------------------+
1 row in set (0.00 sec)

mysql> SELECT SUBSTRING('MySQL', 3); #从第三位开始截取，取到最后
+-----------------------+
| SUBSTRING('MySQL', 3) |
+-----------------------+
| SQL |
+-----------------------+
1 row in set (0.00 sec)

mysql> SELECT SUBSTRING('MySQL', -1);  #从最后一位开始截取，取到最后
+------------------------+
| SUBSTRING('MySQL', -1) |
+------------------------+

10、[NOT] LIKE  # 进行模式的匹配

mysql> SELECT 'MySQL' LIKE 'M%';  # %匹配任意数量的字符， _（下划线）只匹配1个字符
+-------------------+
| 'MySQL' LIKE 'M%' |
+-------------------+
| 1     #代表TRUE |
+-------------------+
1 row in set (0.01 sec)

mysql> SELECT * FROM test;
+----+----------+
| id | username |
+----+----------+
| 1 | John |
| 2 | Mary |
| 3 | 1%Lily |
+----+----------+
3 rows in set (0.00 sec)

mysql> SELECT * FROM test WHERE username LIKE '%1%%' ESCAPE '1';  #1后面的%不需要认为是通配符
+----+----------+
| id | username |
+----+----------+
| 3 | 1%Lily |
+----+----------+