使用 SqlCommand 从多个表中删除

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#.

Answer 2

There is no option in MSSQL Server to remove data from multiple tables in one statement. So, your statement DELETE Posts, Comments FROM is incorrect.

You have two options to set this foreign key to be CASCADE DELETE or to rewrite your statement for deleting data in these two tables like the following one:

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

Answer 3

Your MS SQL query is incorrect. If you want to delete from mutiple tables, you have two choices:

1. setup cascading delete where a delete in one table will automatically cause deletes in dependent tables

2. write separate delete queries such as:

• DELETE FROM Comments WHERE PostId = @PostId
• DELETE FROM Posts WHERE PostId = @PostId

However, I suggest that you test your queries through the SQL Management Studio first before trying to develop code for them.