1.1 MySQL's concat function can connect one or more strings, such as
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)
The concat function of Oracle can only connect two strings.
SQL> select concat('11','22') from dual;
1.2 When the concat function of MySQL connects strings, as long as one of them is NULL, it will return NULL
mysql> select concat('11','22',null);
+------------------------+
| concat('11','22',null) |
+------------------------+
| NULL |
+------------------------+
1 row in set (0.00 sec)
When Oracle's concat function is connected, as long as there is a string that is not NULL, it will not return NULL
SQL> select concat('11',NULL) from dual;
CONCAT
--
11
2. The concat_ws() function, which means concat with separator, that is, a string connection with a separator,
such as a comma-separated connection after the connection
mysql> select concat_ws(',','11','22','33');
+-------------------------------+
| concat_ws(',','11','22','33') |
+-------------------------------+
| 11,22,33 |
+-------------------------------+
1 row in set (0.00 sec)
Unlike concat, the concat_ws function will not return NULL because of the NULL value when it is executed.
mysql> select concat_ws(',','11','22',NULL);
+-------------------------------+
| concat_ws(',','11','22',NULL) |
+-------------------------------+
| 11,22 |
+-------------------------------+
1 row in set (0.00 sec)
3. group_concat() can be used to convert rows to columns. Oracle does not have such a function. The
complete syntax is as follows
group_concat([DISTINCT] The field to be connected [Order BY ASC/DESC sorting field] [Separator 'Separator']) The
following example
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 Group by id, print the value of the name field on one line, separated by commas (default)
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 Group by id, print the value of the name field on one line, separated by semicolons
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 Group by id, print the value of the de-redundant name field on one line, separated by commas
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 Group by id, print the value of the name field on one line, separated by commas, in reverse order by name
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. The repeat() function is used to copy strings. The following 'ab' indicates the string to be copied, and 2 indicates the number of copies to be copied.
mysql> select repeat('ab',2);
+----------------+
| repeat('ab',2) |
+----------------+
| abab |
+----------------+
1 row in set (0.00 sec)
another example
mysql> select repeat('a',2);
+---------------+
| repeat('a',2) |
+---------------+
| aa |
+---------------+
1 row in set (0.00 sec)