Joining is the defining functionality of SQL and sets it apart from other data technologies. Be sure you are somewhat comfortable with the material we’ve covered so far, and take your time practicing and reviewing before moving on.
Let’s rewind back to the beginning of this book, when we were discussing relational databases. Remember how “normalized” databases often have tables with fields that point to other tables? For example, consider this
CUSTOMER_ORDER table, which has a
CUSTOMER_ID field (Figure 8-1).
CUSTOMER_ID field gives us a key to look up in the table
CUSTOMER. Knowing this, it should be no surprise that the
CUSTOMER table also has a
CUSTOMER_ID field (Figure 8-2).
We can retrieve customer information for an order from this table, very much like a VLOOKUP in Excel.
This is an example of a relationship between the
CUSTOMER_ORDER table and the
CUSTOMER table. We can say that
CUSTOMER is a parent to
CUSTOMER_ORDER depends on
CUSTOMER for information, it is a child of
CUSTOMER cannot be a child of