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MySQL Date Functions
Built-In Date Functions In Mysql
NOW()  == Returns the current date and time
CURDATE() == Returns the current date
CURTIME()==Returns the current time
DATE()==Extracts the date part of a date or date/time expression
EXTACT() == Returns a single part of a date/time
DATE ADD () == Adds a specified time interval to a date
DATE SUB() == Subtracts a specified time interval from a date
DATEDIFF () == Returns the number of days between two dates
DATE FORMAT () == Displays date/time data in different formats

Built-In Date Functions In Sql Server:
GETDATE()==Returns the current date and time
DATEPART()==Returns a single part of a date/time
DATEADD()==Adds or subtracts a specified time interval from a date
DATEDIFF()==Returns the time between two dates
CONVERT()==Displays date/time data in different formats

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SQL NULL Values
There is a optional column in a table, then we can insert a new record or update an existing record without adding a value to that null value column. This means that the  particular  field will have the  NULL value.
NULL values are different from the  other values when it is compared.
NULL is used as a placeholder for unknown or inapplicable values.
Compare NULL and 0; they are not equivalent.
Example:
Selecting NULL value column
SELECT name,billno,pay  FROM client WHERE pay IS NULL
Selecting NULL value column
SELECT name,billno,pay  FROM client WHERE pay IS NOT NULL

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