$ mysql -uroot -proot
mysql: [Warning] Using a password on the command line interface can be insecure.
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 591
Server version: 5.7.11 MySQL Community Server (GPL)

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Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> set @str = "abcdxyz";
Query OK, 0 rows affected (0.00 sec)

mysql> select @str;
+---------+
| @str    |
+---------+
| abcdxyz |
+---------+
1 row in set (0.00 sec)

mysql> select length(@str);
+--------------+
| length(@str) |
+--------------+
|            7 |
+--------------+
1 row in set (0.01 sec)

mysql> select substring(@str,1,4);
+---------------------+
| substring(@str,1,4) |
+---------------------+
| abcd                |
+---------------------+
1 row in set (0.00 sec)

mysql> select substring(@str,5,7);
+---------------------+
| substring(@str,5,7) |
+---------------------+
| xyz                 |
+---------------------+
1 row in set (0.00 sec)

需要注意的，mysql 的 substring(@str,1,4) 位置是从 1 开始的。