Question

Sorry for the entry-level database question but I really want to learn this.

I have two tables customer_change and customer_full and I want to select rows from customer_full that same customer(same customer_id) but with different customer_points.

I wrote the following query:

SELECT * 
FROM customer_change a,customer_full b 
WHERE 
    a.ID = b.ID AND 
    a.CUSTOMER_POINTS ！= b.CUSTOMER_POINTS

Now it works. But it will return rows in both tables, how could I change the query to only return the rows in the second table?

Also, the returned rows may contain two rows that have the same IDs, could I modify my query to only include the first row that has this ID?

Could experts offer some help? Thanks!

Answer 1

The SQL inequality operator is <>, not !=. (Apologies: I note that != is supported in at least some implementations! As the comments on the question have pointed out, it seems to be a character set problem. I stand by the rest of my answer, though. :))

That said, I recommend learning about JOIN syntax, rather than joining tables using WHERE criteria. The former lends itself to more-readable queries, and lets you exercise finer control over how the tables are joined.

For example, your query above would be:

SELECT * 
FROM customer_change a
JOIN customer_full b ON a.ID = b.ID AND a.CUSTOMER_POINTS <> b.CUSTOMER_POINTS