applying knowledge of SQL for everyday workplace activities - sql

Question

My question is how to properly write a SQL query for the below highlighted/bold question.
There is a table in HMO database which stores doctor's working
hours.Table has following fields
"FirstName","LastName","Date","HoursWorked". write a sql statement
which retrieves average working hours for period January-March for a
doctor with name Joe Doe.
so far i have
SELECT HoursWorked
FROM Table
WHERE DATE = (January - March) AND
SELECT AVG(HoursWorked) FROM Table WHERE FirstName="Joe",LastName="Doe"*

Answer 1

A few pointers as this sounds like a homework question (which we don't answer for you here, but we can try to give you some guidance).
You want to put all the things you want to return from your select first and you want to have all your search conditions at the end.
So the general format would be :
SELECT Column1,
Column2,
Column3,
FROM YourTable
WHERE Column4 = Restriction1
AND Column5 = Restriction2
The next thing you need to think about is how the dates are formatted in your database table. Hopefully they're kept in a column of type datetime or date (options will depend on the database engine you're using, eg, Microsoft SQL Server, Oracle or MySql). In reality some older databases people use can store dates in all sorts of formats which makes this much harder, but since I'm assuming it's a homework type question, lets assume it's a datetime format.
You specify restrictions by comparing columns to a value, so if you wanted all rows where the date was after midnight on the 2nd of March 2012, you would have the WHERE clause :
WHERE MyDateColumn >= '2012-03-02 00:00:00'
Note that to avoid confusion, we usually try to format dates as "Year-Month-Day Hour:Minute:Second". This is because in different countries, dates are often written in different formats and this is considered a Universal format which is understood (by computers at least) everywhere.
So you would want to combine a couple of these comparisons in your WHERE, one for dates AFTER a certain date in time AND one for dates before another point in time.
If you give this a go and see where you get to, update your question with your progress and someone will be able to help get it finished if you have problems.
If you don't have access to an actual database and need to experiment with syntax, try this site : http://sqlfiddle.com/

Answer 2

you already have the answer written
SELECT AVG(HoursWorked) FROM Table WHERE FirstName="Joe",LastName="Doe"*
you only need to fix the query
SELECT AVG(HoursWorked) as AVGWORKED FROM Table WHERE FirstName='Joe' AND LastName='Doe'
That query will give you the average hours worked for Joe Doe, however you only need to get between some time you add the next "AND", if you are using SQL server you can use the built in function DateFromParts(year,month,day) to create a new Date, or if you are using another Database Engine you can convert a string to a DateColumn Convert(Date,'MM/dd/yyyy')
Example
SELECT AVG(HoursWorked) as AVGWORKED FROM Table WHERE FirstName='Joe' AND LastName='Doe' AND DateColumn between DateFromParts(year,month,day) and Convert(Date,'MM/dd/yyyy')
In the example i showed both approaches (datefromparts for the initial date, and convert(date) for the ending date).