MySQL字元函數的詳細介紹

#CONCAT()和CONCAT_WS()字元連接

mysql> SELECT CONCAT('mysql','5.6');
+-----------------------+
| CONCAT('mysql','5.6') |
+-----------------------+
| mysql5.6              |
+-----------------------+
1 row in set (0.09 sec)

mysql> SELECT CONCAT('mysql','-','5.6');
+---------------------------+
| CONCAT('mysql','-','5.6') |
+---------------------------+
| mysql-5.6                 |
+---------------------------+
1 row in set (0.00 sec)

例如：將使用者的first_name和last_name連接起來成一個字串

mysql> SELECT * FROM tdb_test;
+----+------------+-----------+
| id | first_name | last_name |
+----+------------+-----------+
|  1 | A          | B         |
|  2 | Jack       | Bob       |
|  3 | tom%       | 123       |
+----+------------+-----------+
3 rows in set (0.00 sec)

mysql> SELECT CONCAT(first_name,last_name) AS fullname FROM tdb_test;
+----------+
| fullname |
+----------+
| AB       |
| JackBob  |
| tom%123  |
+----------+
3 rows in set (0.00 sec)

CONCAT_WS()的第一個參數為分隔符，後面的為要連接的字元

mysql> SELECT CONCAT_WS('|','A','B','C');
+----------------------------+
| CONCAT_WS('|','A','B','C') |
+----------------------------+
| A|B|C                      |
+----------------------------+
1 row in set (0.00 sec)
mysql> SELECT CONCAT_WS('-','mysql','5.6');
+------------------------------+
| CONCAT_WS('-','mysql','5.6') |
+------------------------------+
| mysql-5.6                    |
+------------------------------+
1 row in set (0.00 sec)

#FORMAT()數字格式化

mysql> SELECT FORMAT(12560.75,1);
+--------------------+
| FORMAT(12560.75,1) |
+--------------------+
| 12,560.8           |
+--------------------+
1 row in set (0.01 sec)

LOWER()和 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)

LEFT()和RIGHT()

分別有兩個參數，第一個為所用的字串，第二個為取幾位元

例如取得MySQL的前兩個位元

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

例如取得MySQL的後兩個字

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

LENGTH取得字串長度

mysql> SELECT LENGTH('MySQL');
+-----------------+
| LENGTH('MySQL') |
+-----------------+
|               5 |
+-----------------+
1 row in set (0.02 sec)

#LTRIM,RTRIM,TRIM

例如直接用SELECT TRIM( ' mysql ');

會刪除mysql的前後的空格，LTRIM只刪除左邊的空格，RTRIM只會刪除右側的空格。

其他用法：利用TRIM()刪除左右兩側的特定字元。

例如：

mysql> SELECT TRIM(LEADING '?' FROM '??MySQL???');
+-------------------------------------+
| TRIM(LEADING '?' FROM '??MySQL???') |
+-------------------------------------+
| MySQL???                            |
+-------------------------------------+
1 row in set (0.02 sec)
mysql> SELECT TRIM(TRAILING '?' FROM '??MySQL???');
+--------------------------------------+
| TRIM(TRAILING '?' FROM '??MySQL???') |
+--------------------------------------+
| ??MySQL                              |
+--------------------------------------+
1 row in set (0.00 sec)
mysql> SELECT TRIM(BOTH '?' FROM '??MySQL???');
+----------------------------------+
| TRIM(BOTH '?' FROM '??MySQL???') |
+----------------------------------+
| MySQL                            |
+----------------------------------+
1 row in set (0.01 sec)

註：TRIM()只能刪除前導和後續的，不能刪除字元中間的，例如TRIM('My SQL');這個空格就刪不掉。

REPLACE()字串替換

例如將??My??SQL???中的？替換成空

mysql> SELECT REPLACE('??My??SQL???','?','');
+--------------------------------+
| REPLACE('??My??SQL???','?','') |
+--------------------------------+
| MySQL                          |
+--------------------------------+
1 row in set (0.00 sec)

SUBSTRING()字串截取

所含參數，從第幾位截，截取幾位。

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

要注意的是，和程式語言不同的是，第一位是1，不是0。

如果只有從第幾位起，沒有截取幾個，就會，一直截到最後。

mysql> SELECT SUBSTRING('MySQL',3);
+----------------------+
| SUBSTRING('MySQL',3) |
+----------------------+
| SQL                  |
+----------------------+
1 row in set (0.00 sec)

位置也可以為負值（倒數）

mysql> SELECT SUBSTRING('MySQL',-1);
+-----------------------+
| SUBSTRING('MySQL',-1) |
+-----------------------+
| L                     |
+-----------------------+
1 row in set (0.04 sec)

註，只能起始位置可以有負值，但是截取長度不能有負值。

LIKE模式比對（常用於查詢）

mysql> SELECT 'MySQL' LIKE 'M%';
+-------------------+
| 'MySQL' LIKE 'M%' |
+-------------------+
|                 1 |
+-------------------+
1 row in set (0.00 sec)

%是指任一或多位元

mysql> SELECT * FROM tdb_test;
+----+------------+-----------+
| id | first_name | last_name |
+----+------------+-----------+
|  1 | A          | B         |
|  2 | Jack       | Bob       |
|  3 | tom%       | 123       |
+----+------------+-----------+
3 rows in set (0.00 sec)

例如查詢，姓名中包含o的使用者

mysql> SELECT * FROM tdb_test WHERE first_name LIKE '%o%';
+----+------------+-----------+
| id | first_name | last_name |
+----+------------+-----------+
|  3 | tom%       | 123       |
+----+------------+-----------+
1 row in set (0.00 sec)

#例如查詢，名稱中包含%的使用者

mysql> SELECT * FROM tdb_test WHERE first_name LIKE '%%%';
+----+------------+-----------+
| id | first_name | last_name |
+----+------------+-----------+
|  1 | A          | B         |
|  2 | Jack       | Bob       |
|  3 | tom%       | 123       |
+----+------------+-----------+
3 rows in set (0.00 sec)

#

发现显示的是所有查询结果。因为mysql会认为上述的%都为通配符。正确操作如下

mysql> SELECT * FROM tdb_test WHERE first_name LIKE '%1%%' ESCAPE '1';
+----+------------+-----------+
| id | first_name | last_name |
+----+------------+-----------+
|  3 | tom%       | 123       |
+----+------------+-----------+
1 row in set (0.00 sec)

注：%：任意个字符.

_:任意一个字符。

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