Delete from multiple tables with SqlCommand

Question

When I try to delete a whole row from 2 tables with my INNER JOIN I get the error shown at the bottom. I have searched on the internet and could not find the problem so I came here for help.

Here is the code:

    var delete = new SqlCommand("DELETE Posts, Comments FROM Posts INNER JOIN Comments ON Posts.PostId = Comments.PostId WHERE Posts.PostId = @PostId;");
    delete.Parameters.AddWithValue("@PostId", postId);
    _dataAccess.ExecuteQuery(delete);

I am getting an error message:

System.Data.SqlClient.SqlException: 'Incorrect syntax near ','.'

Answer 1

Your Problem in the SQL Statement, it is not valid.

You should divide the statement into two ones:

First Delete Comments, then delete Posts

Sequence is Important

var deleteComments = new SqlCommand("DELETE Comments FROM Posts INNER JOIN Comments ON Posts.PostId = Comments.PostId WHERE Posts.PostId = @PostId;");
        deleteComments.Parameters.AddWithValue("@PostId", postId);
        _dataAccess.ExecuteQuery(deleteComments);
        
var deletePosts = new SqlCommand("DELETE Posts WHERE PostId= @PostId;");
        deletePosts.Parameters.AddWithValue("@PostId", postId);
        _dataAccess.ExecuteQuery(deletePosts);

The other option, using one statement:

var delete = new SqlCommand("DELETE Comments FROM Posts INNER JOIN Comments ON Posts.PostId = Comments.PostId WHERE Posts.PostId = @PostId; DELETE Posts WHERE PostId= @PostId;");
            delete.Parameters.AddWithValue("@PostId", postId);
            _dataAccess.ExecuteQuery(delete);

More Explanation:

Using the Following Prepared SQL Script using SQL Studio (SSMS):

CREATE TABLE Posts (PostId  INT, PostText varchar(20))
CREATE TABLE Comments (CommentId INT, PostId INT, CommentText varchar(20))

INSERT INTO Posts VALUES (1, 'text')
INSERT INTO Comments VALUES (1,1, 'comment here')

when I run your DELETE statement

DELETE Posts, Comments FROM Posts INNER JOIN Comments ON Posts.PostId = Comments.PostId WHERE Posts.PostId = 1

It gives me the same error

When I run

DELETE Comments FROM Posts INNER JOIN Comments ON Posts.PostId = Comments.PostId WHERE Posts.PostId = 1;

DELETE Posts WHERE PostId = 1;

It works fine.

So the rule of thumb in such cases is to use SSMS (MS SQL Studio) to test your SQL statement first and then implement it in C#.