哪个更快，为每个类别获取Count（）或从数据集查询以及如何执行

Question

I am currently trying to get number of articles present in each category(specific ID) to show it beside the category name. This is how it looks like

Now I am trying to assign the label control with count to respective categories. I have two options, by running below select query 6 times for each category and assign it.

Select Count(Cat) from tbl_blg where Cat ='1' //should do so for all the 6 ID's 

Or

SqlCommand cmdcount = new SqlCommand("SELECT Cat from tbl_blg",cnn);
        SqlDataAdapter dacount = new SqlDataAdapter(cmdcount);
        dacount.Fill(dscount);

        int scount = Convert.ToInt32(dscount.Tables["tbl_blg"].AsEnumerable().Count(r => r.Field<int>("Cat") == 1));

---

Now First one will be tedious(6 queries at one go), I believe, correct me if am wrong.

Second one will be better(only one query and assigning it to Dataset) however the problem is it is returning null and throwing null exception...

Which one you think is faster ? if it is second option then is my code correct ? it's returning null but should be returning 2

Answer 1

Rewrite your second query to.

SELECT COUNT(Cat) as Count, Cat FROM tbl_blg
Group by Cat

Gives you the count of how much each category is present. What you are doing now is selecting everything from that table and then do the counting. By doing the group by on query level you limit how much results are being retrieved.

The current query retrieves;

Cat1 
Cat1 
Cat1 
Cat1 
Cat2

And with group by statement you retrieve;

4, Cat1
1, Cat2

Doing multiple query is basically a no-go for this situation. Also EF always adds a bit of overhead on your request, so when it is performance critical you could consider using some more low-level ORM like Dapper or start using SQL/Stored procedures.

Answer 2

You can also use a window function.

SELECT COUNT() OVER(PARTITION BY cat ORDER BY cat) AS 'Count'
FROM tbl_blg

Also if you are going to be calling this code constantly I would highly suggest making it a stored procedure. The benefit of a stored procedure is your sql server doesn't have to recompile the code every time.