SQL Functions are the special functions which are used to perform functional tasks. That function have special prototypes and it built into Oracle Database and are available for use in various appropriate SQL statements. Always Remember SQL Functions user defined functions are written in PL/SQL. If you call a SQL Function with an argument without any data type in the SQL function then that time Oracle attempts to convert the argument to the needed datatype before performing the SQL Function. If you call a SQL Function with a null argument then the SQL Function automatically returns null values. The only SQL Function that do not necessarily follow this behavior are CONCAT, NVL, and REPLACE. In the syntax diagrams for SQL Function arguments are indicated by their datatypes. When the parameter function appears in SQL syntax, replace it with one of the functions described in this section. Functions are grouped by the datatypes of their arguments and their return values.