It drops the students table.
The original code in the school’s program probably looks something like
q = "INSERT INTO Students VALUES ('" + FNMName.Text + "', '" + LName.Text + "')";
This is the naive way to add text input into a query, and is very bad, as you will see.
After the values from the first name, middle name textbox FNMName.Text (which is
Robert'); DROP TABLE STUDENTS; --) and the last name textbox LName.Text (let’s call it
Derper) are concatenated with the rest of the query, the result is now actually two queries separated by the statement terminator (semicolon). The second query has been injected into the first. When the code executes this query against the database, it will look like this
INSERT INTO Students VALUES ('Robert'); DROP TABLE Students; --', 'Derper')
which, in plain English, roughly translates to the two queries:
Add a new record to the Students table with a Name value of ‘Robert’
Delete the Students table
Everything past the second query is marked as a comment:
' in the student’s name is not a comment, it’s the closing string delimiter. Since the student’s name is a string, it’s needed syntactically to complete the hypothetical query. Injection attacks only work when the SQL query they inject results in valid SQL.
Edited again as per dan04’s astute comment