The OVERLAY function changes all characters to lower case. The embedded_string replaces the length in string starting at character position start. If length is not specified, then the embedded_string will replace all characters after start in string.
Format: OVERLAY(string PLACING embedded_string FROM start [FOR length])

Example:           OVERLAY('Dare to be creative' PLACING 'great' FROM 12)

Result: ‘Dare to be great’
> PostgreSQL supports OVERLAY.

> Access, DB2, MySQL, and SQL Server do not support OVERLAY. Instead they use REPLACE.

> Oracle does not support OVERLAY. Instead it uses REGEXP_REPLACE.

The LOWER function changes all characters to lower case. Case changes affect only letters. Spaces, numbers, special characters, and punctuation remain unchanged.
Format: LOWER(string)

Example:           LOWER('Look at THAT!')

Result: ‘look at that!’
> Access does not support LOWER. Instead it uses LCASE.

> DB2, and MySQL, support LOWER as well as LCASE.

> Oracle, PostgreSQL, and SQL Server support LOWER.

The CONVERT function changes the representation of a character string within its character set.

Format: CONVERT(value, USING conversion_char_name)

> Access supports CONVERT with a very different format.

> Oracle supports CONVERT but the format is CONVERT(value, target_char_set, source_char_set).

> PostgreSQL supports CONVERT and conversions can be defined using CREATE CONVERSION.

> SQL Server’s CONVERT is very different and is similar to the CAST function.

> DB2, and MySQL, do not support CONVERT.