Understanding Dapper Query Syntax Issues

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Dapper is a popular .NET library used for querying databases. However, it can be finicky when it comes to query syntax, especially when working with Oracle databases. In this article, we’ll delve into the issues surrounding Dapper’s query syntax and explore how to resolve them.

Background on Dapper Query Syntax

Dapper uses a SQL query builder to construct queries for your database. The query builder takes in parameters and builds a SQL string that can be executed against your database. However, when working with Oracle databases, there are some nuances to consider.

Parameter Placement

In Dapper, parameter placeholders are denoted by the @ symbol. However, in Oracle databases, parameter placeholders should use the :sql syntax. This means that instead of using @RouteId, you would use :RouteId.

SQL vs C# Property Names

Another issue arises when it comes to matching property names between your C# code and your SQL query. In Dapper, the property names in your C# code should match the column names in your SQL query.

Issue with Dapper Query Syntax

The Stack Overflow post you provided highlights a common issue with Dapper query syntax. The user is attempting to use @RouteId as a parameter placeholder, but the column name in their SQL query is ROUTE_ID.

Error Message

The error message returned by Oracle is “ORA-00936: missing expression”. This indicates that the database cannot find an expression to bind the parameter to.

Resolving the Issue

To resolve this issue, you need to adjust either your C# code or your SQL query. Here are some possible solutions:

Solution 1: Adjusting the C# Code

Instead of using new { input.RouteId }, try using new { Id = input.RouteId }. This will ensure that the property name in your C# code matches the column name in your SQL query.

using (OracleConnection connection = new OracleConnection(ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString))
{
    try
    {
        var x = connection.QueryAsync<LocationDto>("Select ROUTE_ID as RouteId,  SCHEDULE_STOP as Location, START_TIME as StartTime From SCHEDULE WHERE ROUTE_ID = :RouteId", new { Id = input.RouteId }).Result.ToList();
        
    }
    catch (Exception ex)
    {
    }
}

Solution 2: Adjusting the SQL Query

Alternatively, you can adjust your SQL query to use the correct parameter placeholder. In this case, you would change :RouteId to :sql. However, keep in mind that using :sql will not actually bind the value to a parameter. Instead, it simply escapes the string.

using (OracleConnection connection = new OracleConnection(ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString))
{
    try
    {
        var x = connection.QueryAsync<LocationDto>("Select ROUTE_ID as RouteId,  SCHEDULE_STOP as Location, START_TIME as StartTime From SCHEDULE WHERE ROUTE_ID = :RouteId", new { input.RouteId }).Result.ToList();
        
    }
    catch (Exception ex)
    {
    }
}

Solution 3: Adjusting Both C# Code and SQL Query

Another option is to adjust both your C# code and your SQL query. In this case, you would change @RouteId to :sql, and also update the property name in your C# code to match the column name in your SQL query.

using (OracleConnection connection = new OracleConnection(ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString))
{
    try
    {
        var x = connection.QueryAsync<LocationDto>("Select ROUTE_ID as RouteId,  SCHEDULE_STOP as Location, START_TIME as StartTime From SCHEDULE WHERE ROUTE_ID = :RouteId", new { RouteId = input.RouteId }).Result.ToList();
        
    }
    catch (Exception ex)
    {
    }
}

Conclusion

Dapper query syntax can be finicky, especially when working with Oracle databases. However, by understanding the nuances of parameter placement and SQL vs C# property names, you can resolve issues like “ORA-00936: missing expression”. Remember to adjust either your C# code or your SQL query to match, depending on which value is more accurate.

Additional Considerations

• When working with Oracle databases, it’s essential to understand the differences between @ and :sql syntax for parameter placeholders.
• Make sure to update both your C# code and SQL query to match property names and column names correctly.
• Use the correct parameter placeholder syntax to ensure accurate binding of values to parameters.

Next Steps

To further improve your understanding of Dapper query syntax, consider exploring additional resources such as official documentation and tutorials. With practice and experience, you’ll become proficient in using Dapper to interact with your database.

Last modified on 2023-07-02