Querying a Table by Filtering Criteria from Rows

Introduction

As developers, we often encounter situations where we need to query data based on specific conditions. In this article, we’ll delve into the world of database queries and explore how to filter a table using multiple criteria in C# with Entity Framework.

Understanding the Problem

The problem presented is an advanced search page that allows users to select multiple options from a checkbox list. These selected options should be filtered from a table where each option is a value. The query should apply an “AND” condition to retrieve data that holds for all the values checked.

To illustrate this, let’s consider an example table:

We want to retrieve data for employees who have both “Manager” and “Employee” roles.

Current Approach

The provided code attempts to filter the table by iterating through each selected option and comparing it with each row:

foreach (var item in UIFilter.SecurityOptions)
{
    securityOverrideList.AddRange(
        secureDbContext.
        SecurityOverride.
        Where(p => p.SECURITY_ROLE_CD.Equals(item)).
        Select(p => p.EMPL_ID));
}

However, this approach does not apply the filter as an “AND” operation because it compares each item individually with the list of rows.

Alternative Approach

To achieve the desired result, we can use a different approach that involves multiple iterations through the selected options:

var potentialEmployees = secureDbContext.SecurityOverride.Where(a => UIFilter.SecurityOptions.Contains(a.SECURITY_ROLE_CD));
var securityOverrideList = potentialEmployees.Select(x => x.EMPL_ID).Distinct();

foreach (var item in UIFilter.SecurityOptions)
{
    var subsetEmployeeIds = potentialEmployees.Where(a => a.SECURITY_ROLE_CD == item).Select(x => x.empl_ID);
    securityOverrideList = securityOverrideList.Where(a => subsetEmployeeIds.Contains(a));
}

This approach first retrieves all employees who match any of the selected roles, then filters the results to only include those who meet each individual role.

Understanding the Query

Let’s break down the query to understand what’s happening:

1. var potentialEmployees = secureDbContext.SecurityOverride.Where(a => UIFilter.SecurityOptions.Contains(a.SECURITY_ROLE_CD));

   • This line retrieves all rows from the SecurityOverride table where the SECURITY_ROLE_CD column matches any of the selected options.

2. var securityOverrideList = potentialEmployees.Select(x => x.empl_ID).Distinct();

   • This line projects only the EMPL_ID column and removes duplicates using the Distinct() method.

3. foreach (var item in UIFilter.SecurityOptions)

   • This loop iterates through each selected option.

4. var subsetEmployeeIds = potentialEmployees.Where(a => a.SECURITY_ROLE_CD == item).Select(x => x.empl_ID);

   • For each selected role, this line retrieves only the rows where the SECURITY_ROLE_CD column matches the current role and projects only the empl_ID column.

5. securityOverrideList = securityOverrideList.Where(a => subsetEmployeeIds.Contains(a));

   • This line filters the securityOverrideList to only include IDs that are present in the subsetEmployeeIds.

Result

The final result is a list of employee IDs who meet all the selected roles.

Conclusion

In this article, we explored how to query a table by filtering criteria from rows using C# and Entity Framework. By applying an “AND” condition, we can retrieve data that meets multiple selection criteria. The alternative approach presented in this article demonstrates a more efficient way to achieve this result.

Best Practices

• When dealing with multiple filters, it’s essential to apply each filter individually to avoid overwriting the results.
• Using Distinct() after selecting projected columns can help remove duplicates and improve performance.
• Always consider indexing and caching strategies for large datasets to optimize query performance.

Troubleshooting

If you encounter issues while implementing this approach, ensure that:

• Your database schema is correct, and the data types match your queries.
• The SecurityOverride table has indexes on the relevant columns to improve query performance.
• You’re using the latest version of Entity Framework and its respective documentation.

Last modified on 2023-08-22