Chapter 8. JOIN
Stitching Tables Together
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).
This 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
. Because CUSTOMER_ORDER
depends on CUSTOMER
for information, it is a child of CUSTOMER
. Conversely, CUSTOMER
cannot be a child of CUSTOMER_ORDER ...
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