How to use SQL substring () string intercept function

The substring function in SQL is used to grab a portion of a field data. The name of this function is not exactly the same in different repositories:

Mysql:substr (), SUBSTRING ()
Oracle:substr ()
SQL server:substring ()
The most commonly used approach is as follows (here we use substr () as an example):

SUBSTR (Str,pos): From <str>, select all characters starting from <pos> position. Note that this syntax is not available on SQL Server.

SUBSTR (Str,pos,len): Starting from <pos> position in <str>, the next <len> characters are selected.

SELECT
SUBSTR (course_designater,6,3) as "COURSE number"
From courses
WHERE course_designater like ' excel% '

Example Two

SELECT
UPPER (SUBSTR (lastname,1,1)) | | LOWER (SUBSTR (lastname,2,29))
As "Lastname"
From Students