SQL Advanced App (date function)

SQL Date

When we work on dates, the hardest task is to ensure that the date you insert is formatted to match the format of the date column in the database.

As long as the data contains only the date parts, running the query will not be a problem. However, if time is involved, the situation is a little more complicated.

Before we discuss the complexity of the date query, let's look at the most important built-in date processing functions.

MySQL Date function

The following table lists the most important built-in date functions in MySQL:

function	Description
Now ()	Returns the current date and time
Curdate ()	Returns the current date
Curtime ()	Returns the current time
DATE ()	Extract date part of date or date/time expression
EXTRACT ()	Returns a separate part of the date/time Press
Date_add ()	Add a specified time interval to a date
Date_sub ()	Subtract a specified time interval from a date
DATEDIFF ()	Returns the number of days between two dates
Date_format ()	Display Date/time in a different format

SQL Server Date function

The following table lists the most important built-in date functions in SQL Server:

function	Description
GETDATE ()	Returns the current date and time
DATEPART ()	Returns a separate part of the date/time
DATEADD ()	Add or subtract a specified time interval in a date
DATEDIFF ()	Returns a time between two dates
CONVERT ()	Display Date/time in a different format

SQL Date Data type

MySQL stores date or date/time values in the database using the following data types:

• DATE-Format YYYY-MM-DD
• DATETIME-format: Yyyy-mm-dd HH:MM:SS
• TIMESTAMP-format: Yyyy-mm-dd HH:MM:SS
• Year-format YYYY or YY

SQL Server uses the following data types to store date or date/time values in the database:

• DATE-Format YYYY-MM-DD
• DATETIME-format: Yyyy-mm-dd HH:MM:SS
• smalldatetime-format: Yyyy-mm-dd HH:MM:SS
• TIMESTAMP-Format: Unique number

SQL Date Processing

If the time part is not involved, then we can easily compare two dates!

Suppose we have the following "Orders" table:

OrderId	ProductName	OrderDate
1	Computer	2008-12-26
2	Printer	2008-12-26
3	Electrograph	2008-11-12
4	Telephone	2008-10-19

Now, we want to select a record from the table above that OrderDate is "2008-12-26".

We use the following SELECT statement:

SELECT * from Orders WHERE orderdate= ' 2008-12-26 '

Result set:

OrderId	ProductName	OrderDate
1	Computer	2008-12-26
3	Electrograph	2008-12-26

Now suppose that "Orders" is similar (note the time section in the "OrderDate" column):

OrderId	ProductName	OrderDate
1	Computer	2008-12-26 16:23:55
2	Printer	2008-12-26 10:45:26
3	Electrograph	2008-11-12 14:12:08
4	Telephone	2008-10-19 12:56:10

If we use the SELECT statement above:

SELECT * from Orders WHERE orderdate= ' 2008-12-26 '

Then we won't get the result. This is because the query does not contain a date for the time part.

Tip: If you want to make queries simple and easier to maintain, don't use the time section in dates!

SQL Advanced App (date function)