Q
Problem solve Get help with specific problems with your technologies, process and projects.

# Aggregate functions in a left outer join

I need to create a simple query in MS Access to calculate employee entitled leave, subtract from the sum of leave taken (number of days) and give the available leave days remaining.

The situation you describe sounds like you have two tables. The first table, employees, has one row per employee, and a column for entitled leave.

```empid  empname          emptitle   empentitled
24    P.H.Boss         Manager         15
36    K.Brown          Analyst         10
44    E.R.Win          Modeller        10
58    H.McGillicuddy   DBA             10```

The second table, leavetaken, has employee leave taken, and let's assume this is accomplished simply by recording the date the employee was away.

```empid  leavedate
24    2002-09-13
36    2002-09-13
58    2002-09-17
58    2002-09-18
58    2002-09-19
24    2002-09-20
36    2002-09-20
58    2002-09-24```

Assuming there's one row per employee per day of leave taken, your query would be:

```select empname
, ( empentitled
- count(leavedate) ) as empremaining
from employee
left outer
join leavetaken
on employee.empid = leavetaken.empid
group
by empname
, empentitled```

This query features a left outer join and an aggregate function. The left outer join is required so that all employees are listed, including any like E.R.Win who had no leave taken. An inner join would omit employees which have no matching rows in the leavetaken table. Grouping is required, to produce the aggregate result for each employee. In this case all we need is a count, since we assumed that each row represents one day.

The trick with using the COUNT() aggregate function to produce the correct count in a left outer join is to use COUNT(column) instead of COUNT(*). All aggregate functions except COUNT(*) ignore nulls. In a left outer join, unmatched rows have nulls in any column from the right table. Therefore, COUNT(leavedate) has to produce a zero for employees that have no matching row from the leavedate table. If we use COUNT(*), we get the count of the number of rows, regardless of whether the employee row has a matching leavetaken row. Since grouping occurs at the employee level, COUNT(*) would yield a 1 for every employee that has no matching rows, but otherwise would produce the right answer when the employee does have matching rows.

Suppose that, instead of one row per day of leave taken, the leavetaken table has a start date and end date:

```
empid  leavestart  leaveend

24    2002-09-13  2002-09-13
36    2002-09-13  2002-09-13
58    2002-09-17  2002-09-19
24    2002-09-20  2002-09-20
36    2002-09-20  2002-09-20
58    2002-09-24  2002-09-24```

Now we must use SUM() instead of COUNT(). We're still after the number of days, which we can get individually for each row by doing a date calculation. Note that when the start and end dates are equal, we assume that's one day of leave, but since subtracting the same date from itself would yield zero, we have to add one:

`leaveend - leavestart + 1`

Fortunately, MS Access automatically produces an answer in days whenever we subtract one date from another, but we could also use the DATEDIFF() function.

So the solution for SUM() would be:

```select empname
, ( empentitled
- sum( case
when leavestart is null
then 0
else leaveend - leavestart + 1
end )
) as empremaining
from employee
left outer
join leavetaken
on employee.empid = leavetaken.empid
group
by empname
, empentitled```

## SearchDataManagement

• ### Pandemic triggered data security movement to DBaaS

Database-as-a-service technology has aided enterprises tasked with keeping data secure with IT professionals working from home ...

• ### Who belongs on a high-performance data governance team?

Putting together a high-quality data governance team can be a challenge. Explore the necessary team members and best practices ...

• ### Soda launches cloud service to improve data observability

Data quality vendor Soda has had a busy 2021, building out new services and raising funding to help organizations identify and ...

• ### 15 data science tools to consider using in 2021

Numerous tools are available for data science applications. Read about 15, including their features, capabilities and uses, to ...

• ### Insightsoftware acquisition of Izenda targets embedded BI

ERP vendor Insightsoftware acquired embedded analytics specialist Izenda on April 15, marking its second purchase of an embedded ...

• ### Data scientist shortage leaves organizations uncertain

Despite their desire to use data science in their decision-making process, some organizations can't find qualified data ...

## SearchSAP

• ### S/4HANA Cloud SaaS ERP: Buying team overview

SAP's multi-tenant SaaS ERP, S/4HANA Cloud, is a viable choice for companies that need ease in their infrastructure management. ...

• ### SAP forms financial services partnership with Dediq

SAP and financial industry investment firm Dediq are forming a new business unit to develop applications that help banks and ...

• ### Unpatched applications threaten SAP security

Cyberattacks are a significant threat to unpatched, unprotected SAP applications, according to a new threat intelligence report ...

## SearchSQLServer

• ### SQL Server database design best practices and tips for DBAs

Good database design is a must to meet processing needs in SQL Server systems. In a webinar, consultant Koen Verbeeck offered ...

• ### SQL Server in Azure database choices and what they offer users

SQL Server databases can be moved to the Azure cloud in several different ways. Here's what you'll get from each of the options ...

• ### Using a LEFT OUTER JOIN vs. RIGHT OUTER JOIN in SQL

In this book excerpt, you'll learn LEFT OUTER JOIN vs. RIGHT OUTER JOIN techniques and find various examples for creating SQL ...

## TheServerSide.com

• ### Incorporate diversity and inclusion in technology design

DEI in technology is about more than creating a diverse workplace. We talked to a few DEI professionals about how teams build ...

• ### Microsoft previews OpenJDK distro to the delight of devs

In a move meant to attract more Java developers to its Azure cloud and further support the Java community, Microsoft launched a ...

• ### Supreme Court ruling on Java APIs eases developer worries

Now that the Supreme Court has ruled for Google over Oracle in their high-stakes copyright battle over Java APIs, developers can ...

## SearchDataCenter

• ### Programmable processor technology for next-gen data centers

The right processing technology can benefit your data center. Learn about advancements in CPU technologies, recent vendor ...

• ### Data processing units accelerate infrastructure performance

DPUs often run on networking packets to move information in the data center, instead of supporting processing workflows. Get an ...

• ### New Intel Ice Lake processors boost performance, security

Intel launches third-generation Xeon Scalable processors that bolster security, accelerate common data center workloads by 46% on...

## SearchContentManagement

• ### OpenText releases Cloud Editions content services updates

OpenText CE 21.2 includes federated document compliance that extends to Microsoft Office 365, along with a revamped content ...

As the pandemic disrupts paper workflows, Adobe courts small business users with simple webforms, digital signatures and payments...

• ### Know when to use a headless CMS and when it's not worth it

Headless CMSes aren't a silver bullet for every circumstance. Evaluate three solid use cases for a headless CMS and three ...

## SearchHRSoftware

• ### Face mask detection a newcomer to employee surveillance

Face mask detection has emerged as another form of employee surveillance technology, and its adoption may be helped indirectly by...

• ### 20 diversity and inclusion tools to power your DEI program

Diversity and inclusion tools shouldn't replace strategy, but these tools can help companies in multiple ways, from tracking ...

• ### DEI tools arrive at critical time for HR

Employers are being held accountable for meeting diversity, equity and inclusion goals, especially executives. HR vendors are ...

Close