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Learn how to implement distinct SQL queries in Flutter's Moor database by using customSelect.
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Mastering Moor with Distinct SQL Queries in Flutter

Using the Moor package for database operations in Flutter is a powerful approach for managing data persistence. However, developers often face challenges when executing complex SQL queries, especially those that require distinct selections from multiple tables. This guide addresses one such challenge—how to select distinct records from two tables using Moor in Flutter.

The Problem: Selecting Distinct Values

When working with relational databases, we sometimes need to retrieve unique entries from joined tables. For instance, consider the following SQL query, which selects distinct LKU_ID and LKU_SYMBOL from the LST_KURS and LST_PRODUCT_CALC tables based on a specified PSET_ID:

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To implement this in Flutter with Moor, you will need to understand how to set up your tables and execute the query properly. Let’s break it down step by step.

Setting Up Your Tables

Before running any queries, ensure your database is correctly set up. Here is how you have defined your tables:

1. LST_KURS Table

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2. LST_PRODUCT_CALC Table

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Defining Your Model Class

To hold the data you retrieve, you’ll need to create a model class. Here’s an example based on the data you want to fetch:

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Implementing the Query in Your DAO

Your Data Access Object (DAO) is where the magic happens. You’ll want to modify your function to perform the custom SQL query using customSelect. Here is how you can implement the aforementioned logic:

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Breakdown of the Function

Query Execution: The customSelect method allows you to run complex SQL queries directly.

Binding Variables: The variables parameter binds your psetId to the query, ensuring security and performance.

Mapping Results: The .map((QueryRow row) section transforms each row of the result set into a KursSymbol object.

Conclusion

In this guide, we explored how to select distinct entries from two tables using the Moor library in Flutter. By carefully setting up your tables and models, and crafting a precise SQL query, you can effectively fetch unique values that fit your application's data needs.

Whether you're a beginner or an experienced developer, mastering these concepts will enhance your data handling capabilities in Flutter. Now, you’re ready to implement Moor for powerful, type-safe database interactions in your next project!