mysqli查询问题。 mysqli_real_escape_string错误

Question

Having trouble with a mysqli query - specifically the WHERE stoname= clause.

This doesn't work:

$result = @mysqli_query($dbc, "SELECT * FROM thedb  WHERE coname='{$_SESSION['user']}' AND stoname='{$_SESSION['store']}' ");

If I echo $_SESSION['store'] then it prints as o\\'store which matches what's the in the database. Yet this doesn't work.

However, if I echo mysqli_real_escape_string($dbc,($_SESSION['store'])) then it prints o\\\\\\'store which is NOT what's in the database. Yet it works.

$result = @mysqli_query($dbc, "SELECT * FROM thedb WHERE user='{$_SESSION['user']}' AND stoname='".mysqli_real_escape_string($dbc,($_SESSION['store']))."' ");

I accept that I have working code, but I'm confused as to why this the case. Can anyone explain what I've done / am doing wrong? Thanks

Answer 1

If your table literally contains o\\'store then mysqli_real_escape_string() has done its job correctly. The function's purpose is to escape a string for safe inclusion inside a SQL statement, not to be an exact literal match for what is actually already in your table.

The table value contains literally \\' . When you use that value directly in the SQL statement, the backslash is misinterpreted as an escape character to the ' rather than as a literal \\ as appears in your table. So the query produces no results because the executed SQL statement is:

# MySQL sees only an escaped ' and no \
SELECT * FROM thedb  WHERE coname='something' AND stoname='o\'store'

...meaning the value of stoname actually compared is just o'store without \\ , because \\ has been discarded by MySQL as an escape character.

So mysqli_real_escape_string() produces a value with two changes.

First, the literal ' in your original string is backslashed escaped as \\' for use in the SQL.

Then, the literal \\ already in your string is itself backslash-escaped so that it can be understood as a literal characer by MySQL rather than an escape character. That results in \\\\ . Combined with the escaped \\' , you now have \\\\\\' .

MySQL receives that string \\\\\\' and is able to correctly interpret it as one literal \\ followed by one literal ' after discarding the extra \\ escape character before each. The condition matches the column's actual value and your query is successful.

# MySQL sees an escaped \ followed by an escaped '
SELECT * FROM thedb  WHERE coname='something' AND stoname='o\\\'store'

About storage...

We don't know much about how your table originally received its value, but I have a hunch it was stored in an escaped form. If the string o\\'store was originally o'store without the \\ , it suggests that an escaped value was inserted in the table. That is not usually done, and is undesirable. Correct use of mysqli_real_escape_string() at the time of data insertion should store the original string rather than an escaped string. Escaping is only done when constructing SQL statements.