SQL placeholder using character versus numeric values

Question

I am trying to get more familiar with placeholders to to run SQL commands from python, but cannot successfully pass character commands. Here I describe a simplified solution that will be scaled up. In python I can create a basic SQL query using a placeholder

sql = 'SELECT * FROM schema.my_tablenLIMIT %s'

And set up to run the command wrapped in a function which takes the sql command plus a value (a limit to the number of rows).

def my_function(query, value):
conn=pg_engine().raw_connection()
with conn.cursor() as conn:
  cursor.execute(query, value)
conn.commit()
return rowcount

This can successfully run if I, for example, do my_function(sql, [10]), but if I try to run my_function(sql, ['ALL']) then I receive an error

invalid input syntax for type bigint

So it is clearly expecting a numeric value; is there a different way of passing a character string in this instance, or another way of addressing this problem?

Thanks

Asked By: Nick Crouch

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Source

Answer 1

A quick example

import psycopg2
from psycopg2 import sql

sql_str = sql.SQL('SELECT * FROM schema.my_table LIMIT {}').format(sql.SQL('ALL'))

print(sql_str.as_string(con))
SELECT * FROM schema.my_table LIMIT ALL

Though to be honest sql.SQL is not escaped.

You can get the same effect doing:

sql_str = sql.SQL('SELECT * FROM schema.my_table LIMIT {}').format(sql.Literal(None))

print(sql_str.as_string(con))
SELECT * FROM schema.my_table LIMIT NULL

This uses sql.Literal which is safer.

Answered By: Adrian Klaver

SQL placeholder using character versus numeric values

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