How to add a new column (Python list) to a Postgresql table?

Question

I have a Python list newcol that I want to add to an existing Postgresql table. I have used the following code:

conn = psycopg2.connect(host='***', database='***', user='***', password='***')
cur = conn.cursor()
cur.execute('ALTER TABLE %s ADD COLUMN %s text' % ('mytable', 'newcol'))
conn.commit()

This added the list newcol to my table, however the new column has no values in it. In python, when I print the the list in python, it is a populated list.

Also, the number of rows in the table and in the list I want to add are the same. I'm a little confused.

Thanks in advance for the help.

ALTER TABLE ... ADD COLUMN ... doesn't add values from list - it only add/create (empty) column. To add values to this column you need UPDATE - learn SQL. — furas
– furas, Commented Oct 3, 2016 at 5:31
@furas, woops, got a little confused when pulling it into Python. In SQL, I've manually added each line in the past. Is there a way to update the values and add the whole list? — dtluther
– dtluther, Commented Oct 3, 2016 at 5:52
You could use for loop to get element from list and execute UPDATE query. — furas
– furas, Commented Oct 3, 2016 at 6:01

Anant Bhardwaj · Accepted Answer · 2016-10-03 06:11:04Z

2

ALTER TABLE only changes table schema -- in your case it will create the new column and initialize it with empty (NULL) values.

To add list of values to this column you can do: UPDATE TABLE <table> SET ... in a loop.

answered Oct 3, 2016 at 6:11

Anant Bhardwaj

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