## 3.6 Examples of Common Queries

3.6.1 The Maximum Value for a Column
3.6.2 The Row Holding the Maximum of a Certain Column
3.6.3 Maximum of Column per Group
3.6.4 The Rows Holding the Group-wise Maximum of a Certain Column
3.6.5 Using User-Defined Variables
3.6.6 Using Foreign Keys
3.6.7 Searching on Two Keys
3.6.8 Calculating Visits Per Day
3.6.9 Using `AUTO_INCREMENT`

Here are examples of how to solve some common problems with MySQL.

Some of the examples use the table `shop` to hold the price of each article (item number) for certain traders (dealers). Supposing that each trader has a single fixed price per article, then (`article`, `dealer`) is a primary key for the records.

Start the command-line tool mysql and select a database:

```shell> `mysql your-database-name`
```

(In most MySQL installations, you can use the database named `test`).

You can create and populate the example table with these statements:

```CREATE TABLE shop (
article INT(4) UNSIGNED ZEROFILL DEFAULT '0000' NOT NULL,
dealer  CHAR(20)                 DEFAULT ''     NOT NULL,
price   DOUBLE(16,2)             DEFAULT '0.00' NOT NULL,
PRIMARY KEY(article, dealer));
INSERT INTO shop VALUES
(1,'A',3.45),(1,'B',3.99),(2,'A',10.99),(3,'B',1.45),
(3,'C',1.69),(3,'D',1.25),(4,'D',19.95);
```

After issuing the statements, the table should have the following contents:

```SELECT * FROM shop;

+---------+--------+-------+
| article | dealer | price |
+---------+--------+-------+
|    0001 | A      |  3.45 |
|    0001 | B      |  3.99 |
|    0002 | A      | 10.99 |
|    0003 | B      |  1.45 |
|    0003 | C      |  1.69 |
|    0003 | D      |  1.25 |
|    0004 | D      | 19.95 |
+---------+--------+-------+
```