使用SqlCommand.ExecuteScalar（）时是否需要放置前1

Question

I've analyzed the code of SqlCommand and see that it works fine (see CompleteExecuteScalar method). But I'm not sure should I use top 1 in the query. For example I have select name from Person and table Person contains a lot of records. Should I modify qry to select top 1 name from Person ? Or I can leave qry as is and it will not affect performance/memory etc? I'm afraid that server can buffer some data before output.

Answer 1

You should definitely put TOP 1 in the query if you want to fetch only one row from the database, regardless of whether you are delivering the data through ExecuteScalar or ExecuteReader for instance.

You should not read source code to come to such conclusion. In fact, the way command is implemented is subject to encapsulation and calling classes should not make decisions based on exact implementation of the command.

Instead, my thinking is that the database is scarce resource that should be used optimally in terms of data throughput and related measures. That includes indicating to the database that exactly one row is wanted and be done with it. Let lower layers, such as command implementation and database itself see how to use that information for the best performance.

Answer 2

ExecuteScalar fetches the first column value of the first row. So, it won't make a difference if you add TOP 1 to your query or not. However, it is recommended that TOP 1 be added if you are expecting duplicate values and you need that one particular value (for the sake of speed). One such plausible scenario is that you are reusing some script that returns a set or rows containing Customer Information about a specific customer, provided, your first column is the CustomerID or any primary key.