PostgreSQL query gives unexpected result

Question

I'm trying to do something extremely simple that works, but not the way I expect it to. I have a database with various tables and for each of those tables, I'm trying to extract the column names from the information schema. I'm using the code below and everything works like a charm (python):

import psycopg2 as pgsql

# code to connect and generate cursor

table = 'some_table_name'

query = 'SELECT column_name FROM information_schema.columns WHERE table_name = %s'

cursor.execute(query, (table,))
result = pd.DataFrame(cursor.fetchall())
print(result)

So far, so good. The problem arises when I replace the query variable with the following:

import psycopg2 as pgsql

# code to connect and generate cursor

table = 'some_table_name'

**query = 'SELECT column_name FROM information_schema.columns WHERE table_name='+table

cursor.execute(query)**
result = pd.DataFrame(cursor.fetchall())
print(result)

If I print the statement, it's correct:

SELECT column_name FROM information_schema.columns WHERE table_name=some_table_name

However, when I run the query, I'm getting this error message:

UndefinedColumn: column "some_table_name" does not exist
LINE 1: ... FROM information_schema.columns WHERE table_name=some_tabl...

some_table_name is a table name as a parameter to the WHERE clause, not a column name. How is this even possible?

Thanks!

Answer 1

Your problem is that you haven't put some_table_name in quotes so it is treated as a column name, not a string literal. Why not stick with the first method which both worked and is in line with the psycopg documentation?