Question

I am trying to extract data that corresponds to a stock that is present in both of my data sets (given in a code below).

This is my data:

#(stock,price,recommendation)
my_data_1 = [('a',1,'BUY'),('b',2,'SELL'),('c',3,'HOLD'),('d',6,'BUY')]

#(stock,price,volume)
my_data_2 = [('a',1,5),('d',6,6),('e',2,7)]

Here are my questions:

Question 1:

I am trying to extract price, recommendation, and volume that correspond to asset ‘a’. Ideally I would like to get a tuple like this:

(u'a',1,u'BUY',5)

Question 2:

What if I wanted to get intersection for all the stocks (not just ‘a’ as in Question 1), in this case it is stock ‘a’, and stock ‘d’, then my desired output becomes:

(u'a',1,u'BUY',5)
(u'd',6,u'BUY',6)

How should I do this?

Here is my try (Question 1):

import sqlite3

my_data_1 = [('a',1,'BUY'),('b',2,'SELL'),('c',3,'HOLD'),('d',6,'BUY')]

my_data_2 = [('a',1,5),('d',6,6),('e',2,7)]

 #I am using :memory: because I want to experiment
 #with the database a lot

conn = sqlite3.connect(':memory:') 

c = conn.cursor()

c.execute('''CREATE TABLE MY_TABLE_1
          (stock TEXT, price REAL, recommendation TEXT )''' )

c.execute('''CREATE TABLE MY_TABLE_2
          (stock TEXT, price REAL, volume REAL )''' )



for ele in my_data_1:
    c.execute('''INSERT INTO MY_TABLE_1 VALUES(?,?,?)''',ele)

for ele in my_data_2:
    c.execute('''INSERT INTO MY_TABLE_2 VALUES(?,?,?)''',ele)    

conn.commit()

# The problem is with the following line:

c.execute( 'select* from my_table_1 where stock = ? INTERSECT select* from my_table_2 where stock = ?',('a','a')  )

for entry in c:
    print entry

I get no error, but also no output, so something is clearly off.

I also tried this line:

c.execute( 'select* from my_table_1 where stock = ? INTERSECT select volume from my_table_2 where stock = ?',('a','a') 

but it does not work, I get this error:

    c.execute( 'select* from my_table_1 where stock = ? INTERSECT select volume from my_table_2 where stock = ?',('a','a')  )
sqlite3.OperationalError: SELECTs to the left and right of INTERSECT do not have the same number of result columns

I understand why I would have different number of resulting columns, but don’t quite get why that triggers an error.

How should I do this?

Thank You in advance

Answer 1

It looks like those two questions are really the same question.

Why your query doesn’t work: Let’s reformat the query.

SELECT * FROM my_table_1 WHERE stock=?
INTERSECT
SELECT volume FROM my_table_2 WHERE stock=? 

There are two queries in the intersection,

1. SELECT * FROM my_table_1 WHERE stock=?
2. SELECT volume FROM my_table_2 WHERE stock=?

The meaning of “intersect” is “give me the rows that are in both queries”. That doesn’t make any sense if the queries have a different number of columns, since it’s impossible for any row to appear in both queries.

Note that SELECT volume FROM my_table_2 isn’t a very useful query, since it doesn’t tell you which stock the volume belongs to. The query will give you something like {100, 15, 93, 42}.

What you’re actually trying to do: You want a join.

SELECT my_table_1.stock, my_table_2.price, recommendation, volume
    FROM my_table_1
    INNER JOIN my_table_2 ON  my_table_1.stock=my_table_2.stock
    WHERE stock=?

Think of join as “glue the rows from one table onto the rows from another table, giving data from both tables in a single row.”

It’s bizarre that the price appears in both tables; when you write the query with the join you have to decide whether you want my_table_1.price or my_table_2.price, or whether you want to join on my_table_1.price=my_table_2.price. You may want to consider redesigning your schema so this doesn’t happen, it may make your life easier.