Understand how to combine the GETDATE function with other functions to write useful queries.
SQL GETDATE, SYSDATE, and TODAY
Nearly every database engine will have a function that returns the current date. You'll need to check the documentation with your engine, but it will typically be GETDATE,SYSDATE, or TODAY.
These functions can be used as part of a separate function call. You can use these functions if you have a value in a column and want to compare it to today's date to determine the difference in the number of days,
weeks, or other time interval that you are interested in.
For example, if you have a column named MyDate in your table named MyTable and want to get the number of weeks between MyDate and today's date, the following statement will do the trick:
SELECT DATEDIFF(week, GETDATE(), MyDate)
This assumes that your engine supports GETDATE.
You will have to substitute the correct function call for your engine. Here is an example of this function using the PUBS database:
SELECT DateDiff(day, getdate(), pubdate) from titles
Some SQL products, including SQL Server, provide access to system values through built-in functions rather than symbolic constants. The SQL Server version of the preceding query is
SELECT NAME, HIRE_DATE
WHERE HIRE_DATE > GETDATE();
In Oracle, the datetime system function is SYSDATE. Oracle 9i and later versions support CURRENT_DATE and CURRENT_TIMESTAMP.
SELECT SYSDATE AS "Date"
SYSDATE returns the system date and time but does not display the time unless formatted to do so with the function
➝ ’YYYY-MM-DD HH24:MI:SS’)
A number of useful built-in functions are specified in SQL standard, and most SQL implementations add many more. These facilities often provide data type conversion facilities. For example, DB's built-in MONTH() and YEAR() functions take a DATE or TIMESTAMP value as their input and return an integer that is the month or year portion of the value. This query lists the name and month of hire for each salesperson in the sample database:
SELECT NAME, MONTH(HIRE_DATE)
and this one lists all salespeople hired in 2016:
SELECT NAME, MONTH(HIRE_DATE)
WHERE YEAR(HIRE_DATE) = 2016;
Built-in functions are also often used for data reformatting. Oracle's built-in TO_CHAR() function, for example, takes a DATE data type and a format specification as its arguments and returns a string containing a formatted character string version of the date.
(This same function is also capable of converting numeric values to formatted character strings.) In the results produced by this query:
SELECT NAME, TO_CHAR(HIRE_DATE,'DAY MONTH DD, YYYY')
the hire dates will all have the format "Wednesday June 14, 2017" because of the built-in function. In general, a built-in function can be specified in a SQL expression anywhere that a constant of the same data type can be specified. The built-in functions supported by popular SQL dialects are too numerous to list here.
The IBM DB2 SQL dialects include about two dozen built-in functions, Oracle supports a different set of about two dozen built-in functions, and SQL Server has several dozen.
The SQL2 standard incorporated the most useful built-in functions from these implementations, in many cases with slightly different syntax. These functions are summarized in Table 5-16.
Sql Date - Quiz
Take this brief quiz to make sure you understand the date functions. Sql Date - Quiz