Joe Celko's SQL for Smarties, 3rd Edition

29.1. Temporal Math

Almost every SQL implementation has a DATE data type, but the functions available for that data type vary quite a bit. The most common ones are a constructor that builds a date from integers or strings; extractors to pull out the month, day, or year; and some display options to format output.

You can assume that your SQL implementation has simple date arithmetic functions, although with different syntax from product to product, such as:

1.	A date plus or minus a number of days yields a new date
2.	A date minus a second date yields an integer number of days

Table 29.1 shows the valid combinations of <datetime> and <interval> data types in Standard SQL:

Table 29.1. Valid Combinations of Temporal Data Types in Standard SQL

<datetime> ...