Question

I have two table user and contact, each user can have multiple contacts
I want to draw a graph (camembert) with JFreeChart(java library) to present the number of contacts that has each user
I tested :

select u.name, sum(c) from user u, contact c

but I have

unknown column c in field list

here are the structures of tables :

utilisateur = idutilisateur, identifiant, motdepasse, nom, prenom

contact = idcontact, …………, idutilisateur

How can I achieve that,

and also, please give me links to tutorials for learning to make queries like that because always I have this problem and I find only basic tutorials
thank you in advance

Answer 1

You need to reference the column of the table that you’re trying to count, not just the table itself.

if you try:

select u.name, count(c.ID) 
from user u, contact c

You can count all records that have a value in the ID field. If this is a primary key, then all records should have this field. Note the change from sum to count.

This won’t fully solve your problem. You also have to let sql know that the two tables are related by joining:

JOIN contact
ON user.ID=contact.userID

Put this after the from user u instead of , contact c

For a good start to learning sql, try here: http://www.w3schools.com/sql/default.asp