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Calculating time intervals

From: Access 2010: Queries in Depth

Video: Calculating time intervals

Now that we know how to tell Access to return today's date and time, we can use that information to help obtain query results that are relative to today. We have two different functions that will return slightly different results depending on the type of question that we're trying to ask. The DateAdd function will allow us to specify a time frame such as the past 30 days or within the last year and return records within that timeframe. The DateDiff function will allow us to find how many days have elapsed between today and a specific event. We'll take a look at how we can apply each in our queries with some real world examples.

Calculating time intervals

Now that we know how to tell Access to return today's date and time, we can use that information to help obtain query results that are relative to today. We have two different functions that will return slightly different results depending on the type of question that we're trying to ask. The DateAdd function will allow us to specify a time frame such as the past 30 days or within the last year and return records within that timeframe. The DateDiff function will allow us to find how many days have elapsed between today and a specific event. We'll take a look at how we can apply each in our queries with some real world examples.

First let's take a look at the DateAdd function. We'll go to the Create menu and in Query Design and we'll add our table Orders. Go ahead and say Close to the Show Table window and in the first field let's just put in the Field for today's date, date(). Now let's add a DateAdd function. In the next field we'll right-click and say Build. We'll go into the Functions, the Built- In Functions, and the Date/Time category.

Let's scroll until we find DateAdd. We'll double-click on it to add it to our function. So DateAdd requires three parts. The first is an interval. This is if they were looking for months or days or years. The second part is a number. This is how many months or days or years to add or subtract. Finally we need the date that we want to add or subtract from. So if we wanted to find a date that was 24 months ago, we would type it like this. The interval is months, so we'll use the m character.

The number that we want to add is -24. We want 24 months ago. The date that we want to add or subtract from is today. That will use the date function to get. So our finished function reads DateAdd("m", -24, date(), and a final closing parenthesis to finish the function. Okay, let's go ahead and say OK and run our query.

We'll say that Access returns today's date and the same day two years ago or 24 months ago. Let's go ahead and go back into Design view and change it up. I'll expand this open and now once it's created we can just edit it right here. Instead of 24 months ago, let's say one day in the future. We'll change it to a d to indicate that we want to add days instead of months. We'll change the number to a 1 so we'll add one day to today's date. Let's go ahead and run that and we'll see that we get today and tomorrow.

Let's take a look at another one. Let's go back into Design view. This time we're going to add years. Now years is going to use four y's, similar to the Format function symbols. Let's add 3 years to today's date. We'll change that 1 to a 3 and then we'll run our query. Now I've got today's date and a date three years from now. So now that we know how to use today's date and we know how to specify the date in the future or in the past relative to today, we can use these two to define a range.

Let's go back into Design view. Let's use these two fields to create a function that tells us what employees were hired in the last two years. Let's go ahead and delete the Orders table and we'll delete these two fields to get us back to a blank query. We'll Show Table and we'll add our Employees table. Go ahead and say Close. To our query we're going to add FirstName, LastName, and we'll scroll down until we get to HireDate and we'll add that as well.

For HireDate we'll specify a criteria that selects everything more recent than two years ago. Let's expand this field open. In our Criteria we'll say it's greater than the dateadd function, the units or the interval is going to be in years, and the number is going to be -2. We want a date that's two years ago. And finally the day that we want to add or subtract that to is today, date().

Finally let's finish our function with a finishing closing parenthesis. So the finished criteria reads that we want to select all of our dates that are greater than a day two years in the past. Let's go ahead and run this query and we'll see that we have 20 employees they were hired within the last two years. So this is an example of how we can use DateAdd to select a range of dates. Let's find out exactly how many days ago some of these employees were hired. Let's go back into our Design view and we'll use the DateDiff function. In our fourth column we'll right-click and go to our Builder and let's go find the DateDiff function.

We'll look in Built-In Functions, Date/Time and DateDiff. DateDiff has several arguments that we can supply. The first one is interval. This is going to be the same as with DateAdd, whether we're looking at days, months, or years. The second and third arguments are the two dates that we want to find the difference between. The firstdayofweek and the firstweekofyear fields are optional. We can see that if we look at the hints down here in the bottom. They're wrapped in square brackets which is Access's way of telling you that these are optional arguments. We would use these if we needed to redefine when our week started or when our year started. For instance, if we are on a fiscal year that doesn't coincide with the calendar year.

We can go ahead and remove these. We're not going to be using them. But we do need to supply though is the interval and in the two dates that we want to calculate the difference between. So the interval for our example is going to be months. We want to find out how many months has it been since our employees were hired. We'll wrap that in quotation marks so that we maintain proper syntax. Then we need to specify the two dates. The first date that we want to use is the HireDate. So we'll go into our database, we'll go into our Tables, we'll go into our Employees table, and we'll double-click on HireDate.

The date we want to find the difference between is today so we'll use our code to specify today's date, date(). Let's go ahead and say OK and we'll run this query. Now we had a query that's showing us all of the employees they were hired within the last two years and in this fourth column we're seeing exactly how many months ago from the date of this recording that they were hired. Working with dates is an integral part of making sense of all the data within your database.

With a little bit of practice and persistence, mastering date and time-based data and using it to filter out all the relevant records will help you make your queries that much more successful.

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This video is part of

Image for Access 2010: Queries in Depth
Access 2010: Queries in Depth

46 video lessons · 13458 viewers

Adam Wilbert
Author

 
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  1. 9m 9s
    1. Welcome
      1m 10s
    2. Using the exercise files
      41s
    3. Introducing the database
      4m 29s
    4. Previewing the course
      2m 49s
  2. 17m 17s
    1. Understanding queries
      3m 31s
    2. Following naming conventions and best practices
      2m 56s
    3. Using the Query Wizard
      5m 21s
    4. Exploring the design interface
      5m 29s
  3. 26m 39s
    1. Defining criteria
      5m 40s
    2. Understanding comparison operators
      3m 19s
    3. Defining the column headers
      2m 49s
    4. Exploring the property sheet
      7m 32s
    5. Printing query results
      2m 41s
    6. Working with joins
      4m 38s
  4. 14m 14s
    1. Understanding parameter queries
      4m 27s
    2. Obtaining parameters from forms
      5m 17s
    3. Creating a combo box
      4m 30s
  5. 23m 24s
    1. Understanding the Totals field
      5m 31s
    2. Creating aggregate calculations
      3m 31s
    3. Exploring the Expression Builder interface
      4m 28s
    4. Using mathematical operators
      5m 46s
    5. Applying text functions
      4m 8s
  6. 24m 23s
    1. Understanding dates as serial numbers
      2m 42s
    2. Specifying a range of dates or times
      3m 47s
    3. Formatting dates
      4m 31s
    4. Using other Date/Time functions
      3m 47s
    5. Defining today's date
      2m 41s
    6. Calculating time intervals
      6m 55s
  7. 20m 9s
    1. Introducing the conditional IIf function
      2m 57s
    2. Creating an IIf function
      7m 31s
    3. Nesting IIf functions
      4m 57s
    4. Using the Switch function
      4m 44s
  8. 20m 41s
    1. Understanding the reporting tool
      2m 13s
    2. Building the form
      6m 57s
    3. Building the query
      5m 4s
    4. Building the report
      3m 30s
    5. Finalizing the reporting tool
      2m 57s
  9. 25m 37s
    1. Finding duplicate records
      2m 17s
    2. Identifying unmatched records
      2m 29s
    3. Creating crosstab results
      2m 57s
    4. Creating backups
      1m 29s
    5. Creating update queries
      3m 22s
    6. Making, deleting, and appending records
      5m 36s
    7. Uniting tables
      3m 16s
    8. Embedding SQL code in queries
      4m 11s
  10. 1m 0s
    1. Next Steps
      1m 0s

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