對(duì)照Oracle，MySQL的幾個(gè)使用字符串函數(shù)

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1、concat()函數(shù)
1.1 MySQL的concat函數(shù)可以連接一個(gè)或者多個(gè)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函數(shù)只能連接兩個(gè)字符串
SQL> select concat('11','22') from dual;

1.2 MySQL的concat函數(shù)在連接字符串的時(shí)候，只要其中一個(gè)是NULL,那么將返回NULL
mysql> select concat('11','22',null);
+------------------------+
| concat('11','22',null) |
+------------------------+
| NULL |
+------------------------+
1 row in set (0.00 sec)

而Oracle的concat函數(shù)連接的時(shí)候，只要有一個(gè)字符串不是NULL,就不會(huì)返回NULL
SQL> select concat('11',NULL) from dual;

CONCAT
--
11

2、concat_ws()函數(shù), 表示concat with separator,即有分隔符的字符串連接
如連接后以逗號(hào)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函數(shù)在執(zhí)行的時(shí)候,不會(huì)因?yàn)镹ULL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()可用來(lái)行轉(zhuǎn)列, Oracle沒(méi)有這樣的函數(shù)
完整的語(yǔ)法如下
group_concat([DISTINCT] 要連接的字段 [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分組，把name字段的值打印在一行，逗號(hào)分隔(默認(rèn))
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分組，把name字段的值打印在一行，分號(hào)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分組，把去冗余的name字段的值打印在一行，逗號(hào)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分組，把name字段的值打印在一行，逗號(hào)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()函數(shù)，用來(lái)復(fù)制字符串,如下'ab'表示要復(fù)制的字符串，2表示復(fù)制的份數(shù)
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)

..

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