GREATEST returns the greatest value from a comma-separated series of expressions. Expressions are evaluated in left-to-right order. If only one expression is provided, GREATEST returns that value. If any expression is NULL, GREATEST returns NULL.
If all of the expression values resolve to canonical numbers, they are compared in numeric order. If a quoted string contains a number in canonical format, it is compared in numeric order. However, if a quoted string contains a number not in canonical format (for example, '00', '0.4', or '+4'), it is compared as a string. String comparisons are performed character-by-character in collation order. Any string value is greater than any numeric value.
The empty string is greater than any numeric value, but less than any other string value.
If the returned value is a number, GREATEST returns it in canonical format (leading and trailing zeros removed, etc.). If the returned value is a string, GREATEST returns it unchanged, including any leading or trailing blanks.
GREATEST returns the greatest value from a comma-separated series of expressions. LEAST returns the least value from a comma-separated series of expressions. COALESCE returns the first non-NULL value from a comma-separated series of expressions.
Data Type of Returned Value
If the data types of the expression values are different, the data type returned is the type most compatible with all of the possible return values, the data type with the highest data type precedence. For example, if one expression is an integer and another expression is a fractional number, GREATEST returns a value with data type NUMERIC. This is because NUMERIC is the data type with the highest precedence that is compatible with both. If, however, an expression is a literal number or string, GREATEST returns data type VARCHAR.
In the following example, each GREATEST compares three canonical numbers:
SELECT GREATEST(22,2.2,-21) AS HighNum,
GREATEST('2.2','22','-21') AS HighNumStr
In the following example, each GREATEST compares three numeric strings. However, each GREATEST contains one string that is non-canonical; these non-canonical values are compared as character strings. A character string is always greater than a number:
In the following example, each GREATEST compare three strings and returns the value with the highest collation sequence:
The following example compares two dates, treated as canonical numbers: the date of birth as a $HOROLOG integer, and the integer 58073 converted to a date. It returns the date of birth for each person born in the 21st century. Anyone born before January 1, 2000 is displayed with the default birth date of December 31, 1999:
SELECT Name,GREATEST(DOB,TO_DATE(58073)) AS NewMillenium