3.3.4.9 Using More Than one Table

The pet table keeps track of which pets you have. If you want to record other information about them, such as events in their lives like visits to the vet or when litters are born, you need another table. What should this table look like? It needs:

• To contain the pet name so you know which animal each event pertains to.
• A date so you know when the event occurred.
• A field to describe the event.
• An event type field, if you want to be able to categorise events.

Given these considerations, the CREATE TABLE statement for the event table might look like this:

mysql> CREATE TABLE event (name VARCHAR(20), date DATE,
    -> type VARCHAR(15), remark VARCHAR(255));

As with the pet table, it's easiest to load the initial records by creating a tab-delimited text file containing the information:

Load the records like this:

mysql> LOAD DATA LOCAL INFILE "event.txt" INTO TABLE event;

Based on what you've learned from the queries you've run on the pet table, you should be able to perform retrievals on the records in the event table; the principles are the same. But when is the event table by itself insufficient to answer questions you might ask?

Suppose you want to find out the ages of each pet when they had their litters. The event table indicates when this occurred, but to calculate the age of the mother, you need her birth date. Because that is stored in the pet table, you need both tables for the query:

mysql> SELECT pet.name,
    -> (TO_DAYS(date) - TO_DAYS(birth))/365 AS age,
    -> remark
    -> FROM pet, event
    -> WHERE pet.name = event.name AND type = "litter";
+--------+------+-----------------------------+
| name   | age  | remark                      |
+--------+------+-----------------------------+
| Fluffy | 2.27 | 4 kittens, 3 female, 1 male |
| Buffy  | 4.12 | 5 puppies, 2 female, 3 male |
| Buffy  | 5.10 | 3 puppies, 3 female         |
+--------+------+-----------------------------+

There are several things to note about this query:

• The FROM clause lists two tables because the query needs to pull information from both of them.
• When combining (joining) information from multiple tables, you need to specify how records in one table can be matched to records in the other. This is easy because they both have a name column. The query uses WHERE clause to match up records in the two tables based on the name values.
• Because the name column occurs in both tables, you must be specific about which table you mean when referring to the column. This is done by prepending the table name to the column name.

You need not have two different tables to perform a join. Sometimes it is useful to join a table to itself, if you want to compare records in a table to other records in that same table. For example, to find breeding pairs among your pets, you can join the pet table with itself to pair up males and females of like species:

mysql> SELECT p1.name, p1.sex, p2.name, p2.sex, p1.species
    -> FROM pet AS p1, pet AS p2
    -> WHERE p1.species = p2.species AND p1.sex = "f" AND p2.sex = "m";
+--------+------+--------+------+---------+
| name   | sex  | name   | sex  | species |
+--------+------+--------+------+---------+
| Fluffy | f    | Claws  | m    | cat     |
| Buffy  | f    | Fang   | m    | dog     |
| Buffy  | f    | Bowser | m    | dog     |
+--------+------+--------+------+---------+

In this query, we specify aliases for the table name in order to refer to the columns and keep straight which instance of the table each column reference is associated with.

User Comments

For all newbies like me - the helpful code
explanations sort of fade away at this point in the
tutorial. I don't know why - this is the point at which
the queries start to get a lot more challenging.
TO_DAYS(MM-DD-YYYY) counts the number of days
from year 0 to the date given in brackets. Don't use
it for dates prior to 1582 because the function
doesn't take into account the number of days lost
due to the change to the Gregorian calendar.

Re date functions and the Gregorian Calendar.

You have to think about using the date function for much more recent dates than 1582. The change from the Julian to Gregorian calendar was not made in most Protestant countries until much later.

For example, Britain and its "dominions" (including what is now the US) kept to Julian calendar until 1751/2 when it was subject to the British Calendar Act of 1751. Until then, each year started on March 25. The Calendar Act made the following changes: 1751 ran from 25 March 1751 until 31 December (a short year) when the change to January 1 as "the new year" was made. Then 1752 had 11 days chopped out (there was no Sept 3 - Sept 13) to bring things into line with Rome.

Some countries didn't adopt the Gregorian Calendar until the 20 Century (Alaska changed in around 1865).

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