sql-where

C# – Select a single row with Dapper

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When you want to select a single row with Dapper, the simplest option is to use QuerySingleOrDefault() with a SELECT + WHERE query (and pass in the parameters / model type like usual). Here’s an example: This SQL query returns a single row from the Movies table and Dapper maps it and returns a Movie … Read more

C# – Update records with Dapper

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You can update records with Dapper by using Execute() with an UPDATE statement and passing in the parameters. Here’s an example: Dapper maps the properties from the param object to the query parameters (ex: it maps year to @year). You can pass in the parameters with an anonymous type (as shown above) or by passing … Read more

C# – Delete records with Dapper

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You can delete records with Dapper by using Execute() with a DELETE statement and specifying the record ID as a parameter. Here’s an example: This deletes a single record from the Movies table. Delete multiple records When you’re deleting multiple records, you can efficiently execute a single DELETE statement with a WHERE clause including all … Read more

C# – Execute a stored procedure with Dapper

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You can execute stored procedures with Dapper by specifying the name, parameters, and CommandType.StoredProcedure. Let’s say you have the following stored procedure (single parameter, returns movie rows): Here’s how to execute this stored procedure with Dapper and map its results to Movie objects: Use an output parameter To get a stored procedure’s output parameter, add … Read more

SQL Server – Copy data from one table to another

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To copy data from one table to an existing table, use INSERT INTO SELECT and specify the column list: If the columns are the exact same in the two tables (and there are no identity columns), you don’t need to specify the column list: When the table doesn’t exist, use SELECT INTO, specifying which columns … Read more

C# – Adding dynamic parameters with Dapper

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The simplest way to add dynamic parameters when executing queries with Dapper is by passing in Dictionary<string, object>, like this: Read more about how to add items to a dictionary. You can also add dynamic parameter by using the DynamicParameters class. You can use whichever approach is simplest in the given scenario. In this article, … Read more

C# – Map query results to multiple objects with Dapper

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When you’re querying joined tables, you can map each row to multiple objects by using the multi mapping feature in Dapper. To multi map, you have to provide Dapper with the following: In this article, I’ll show examples of multi mapping. Note: If you don’t specify the split column, it’ll use the default of “Id”. … Read more

C# – Execute a SELECT query with Dapper

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You can query the database with Dapper by using Query() with a SELECT, specifying the type to map the results to, and optionally adding parameters. Here’s an example: Note: It returns an empty list when there’s no results. Dapper abstracts away the repetitive code involved in executing SQL queries, including mapping parameters and query results. … Read more

EF Core – Inheritance mapping

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There are two ways to do inheritance mapping in EF Core: Let’s say we have a database with employees. All employees have an id and a name. There are currently two types of employees: programmers and drivers. Programmers have a language (ex: C#), and drivers have a car (ex: Honda). We can model this with … Read more

C# – How to match an anonymous type parameter in a mocked method

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When an anonymous type is defined in one assembly, it won’t match an anonymous type defined in another assembly. This causes problems when you’re unit testing and trying to mock a method that has an anonymous type parameter. For example, let’s say you’re trying to unit test the following method: To unit test this, you … Read more

Get SQL Server query results as JSON

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The simplest way to get query results as JSON is to use FOR JSON PATH in the query (note: this was added in SQL Server 2016): It returns the results as a single JSON string with one JSON object per row: Note: SQL Server returns the JSON without indenting. All examples in this article show … Read more

SQL – Select from information_schema.columns

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You can query the information_schema.columns system view to get information about all columns in the database. Here’s an example: Here’s the first few rows this returns: TABLE_NAME COLUMN_NAME Movies Id Movies Title Movies Year In this article, I’ll show more examples of querying information_schema.columns. Get columns filtered by name Let’s say you want to find … Read more

Case sensitivity in SQL Server

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In SQL Server, the collation property controls case sensitivity. Case sensitivity affects sorting and queries (even the column names must match exactly if you’re using a case-sensitive collation at the database-level). The default collation when you create a database in SQL Server is SQL_Latin1_General_CP1_CI_AS. The CI stands for case-insensitive, which means SQL Server databases are … Read more

EF Core – Aggregate SELECT queries

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In this article, I’ll show how to use EF Core to aggregate data for the whole table, per group, and how to only include groups that meet a condition. I’ll show three different SQL aggregate functions – count, sum, and average. In each scenario, I’ll show the LINQ query, the SQL query it generated, and … Read more

EF Core – SELECT queries involving multiple tables

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When you’ve created tables that are related, you’ll often need to get data from both tables at once, or filter records from one table based on values in another table. In this article, I’ll show examples of executing queries like this where more than one table is involved. You can do most queries using LINQ. … Read more

EF Core – Basic SELECT queries

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In this article, I’ll show examples of how to execute basic SELECT queries when using EF Core. You can execute queries using LINQ or by writing raw SQL. I’ll use SQL Profiler to show the queries generated by LINQ. Note: I’ll be using .AsNoTracking().ToListAsync() in all cases. You’ll need to decide if that’s the right … Read more

Cannot drop database because it is currently in use

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Problem When you try to drop the database, you get the following error and the drop fails: Cannot drop database because it is currently in use. This means there are other open connections on the database and it won’t let you drop the database. Disclaimer: The solutions in this article involve kicking out other active … Read more

C# – How to unit test code that uses Dapper

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Dapper makes your code difficult to unit test. The problem is that Dapper uses static extension methods, and static methods are difficult to mock out. One approach is to wrap the Dapper static methods in a class, extract out an interface for that wrapper class, and then dependency inject the wrapper interface. In the unit … Read more

SQL Server – Bad SELECT query performance due to missing indexes

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When you have a slow SELECT query, the most likely cause of the problem is that you’re missing an index. When your table has lots of data, having the right indexes will make a difference in the performance. Without indexes, SQL Server has to scan through your entire table to find the rows that match … Read more

Modifying JSON in SQL Server

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There are two ways to modify JSON data in a SQL Server table: In this article, I’ll show how to insert, update, and delete JSON data by using the JSON_MODIFY() function. I’ll also show how to deal with JSON arrays. Note: This is part 2 of the mini-series about the SQL Server JSON API. The … Read more

Querying JSON in SQL Server

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In SQL Server 2016, Microsoft added support for handling JSON data: The JSON API in SQL Server is well-suited for simple scenarios where you only need to do basic querying and manipulation. For advanced querying, the SQL Server JSON API gets complicated and hard to use, so you may want to look into other options. … Read more

Using sp_msForeachTable in SQL Server

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You can use the sp_msForeachTable stored proc in SQL Server to execute a query/command against all tables in the database. To give you an idea about how to use this, here’s an example of using it to do a SELECT query against all tables: Internally, sp_msForeachTable gets all user table names (from dbo.sysobjects) and opens … Read more

SQL – Filtering GROUP BY with HAVING and WHERE

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There are two ways to filter GROUP BY results: In other words, WHERE controls which rows will be considered for grouping, GROUP BY groups the filtered rows, and then HAVING filters the groups. I’ll show examples below. Example – Using WHERE with GROUP BY Let’s say you have the following table and you want to … Read more

C# – How to use IN with Dapper

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Let’s say you have a SQL Query that uses IN and you want to execute it using Dapper. Your query looks something like this: Here’s how you’d execute that with Dapper: Then you’d call it like this: There are two key things to notice about this: Exclude the parentheses In a normal SQL Query you … Read more