I searched for a long time for the answer and did the following:
(1)
query = "SELECT COUNT(*) FROM %s WHERE user_name = %s" % (table_name, username)
result = conn.query(query).result()[0][0]
if result == 0:
(do something)
(2)
query = "SELECT 1 FROM %s WHERE user_name = %s" %(table_name, username)
result = conn.query(query).result()[0][0]
if result == ' ':
(do something)
(3)
query = "SELECT EXISTS (SELECT 1 FROM %s WHERE user_name = %s)" %(table_name, username)
result = conn.query(query).result()[0][0]
if result == 't':
(do something)
But all don't work... the error is always:
column "Tom" does not exist
Since it really doesn't exist and I just want to check if it exists. Any help is appreciated.
sql = """SELECT count(*)
FROM information_schema.columns
WHERE table_name = '%s'
AND column_name = '%s'
""" % (thetable,thecolumn)
You're not quoting your strings.
You query looks like this, once it gets to PostgreSQL:
SELECT COUNT(*) FROM Table WHERE user_name = Tom
Which compares the user_name
column against the non-existant Tom
column.
What you want is a query like this:
SELECT COUNT(*) FROM Table WHERE user_name = 'Tom'
To do this right, you should be using parameterized statements, to avoid any possibility of SQL injection. With a DBAPI module, it's a simple as:
cursor = conn.cursor()
cursor.execute('SELECT COUNT(*) FROM Table WHERE user_name = %s', user_name)
If you need the table name to be dynamic as well, then you have to construct it like this:
cursor = conn.cursor()
query = 'SELECT COUNT(*) FROM %s WHERE user_name = %%s' % table_name
cursor.execute(query, user_name)
But you need to be absolutely certain that the table name is valid. Anybody who could control that variable could do absolutely anything to your database.
Always remember: EAFP
try:
result = conn.query(query).result()
except ProgrammingError: # Edumacated guess based on the documentation
# however untested so psycopg2 raises some other instead
pass # or do_something_when_it_does_not_exist()
else:
(do something)
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