Choosing columns

SQL · Unit 1: First queries

Free
SQL
11 exercises

The store is sending out a notice and only needs the names. With *, each person's city and age would come along too, and nobody asked for those.

Instead of *, write the column you want. You get the same rows, with that one value.

SELECT name
FROM customers;

Returns

name
Ann
Luke
Chloë
Mason
Valerie
Dylan
Hannah
Brian

A price without its product's name is no use to anyone. To see several columns, separate them with commas.

No comma after the last one: what comes next is the FROM.

SELECT name, price
FROM products;

Returns

name	price
Keyboard	45.5
Mouse	19.9
Monitor	189
Cable	5.25
Laptop	899
Tablet	320
Headphones	59.9
Speaker	74

Columns come out in the order you ask for them, not the order the table has them in. You design the result.

In customers, name comes before age. Here they come out the other way around because that's how they were asked for.

SELECT age, name
FROM customers;

Returns

age	name
34	Ann
27	Luke
41	Chloë
19	Mason
52	Valerie
27	Dylan
NULL	Hannah
38	Brian

SELECT * is for taking a peek at a table. In a query that's going to stick around, like the one behind a report, name the columns.

The reason is that tables change. If someone adds a column tomorrow, your * starts returning it too, and nobody decided that.

When there are a lot of columns, put one per line, with two spaces in front. It reads from top to bottom, and removing or adding one means touching a single line.

SQL also doesn't care whether you write SELECT or select. The uppercase is for whoever reads it: at a glance it separates SQL's words from your names.

select name,
  city,
  age
from customers;

Returns

name	city	age
Ann	Denver	34
Luke	Austin	27
Chloë	NULL	41
Mason	Tampa	19
Valerie	Denver	52
Dylan	Boston	27
Hannah	Austin	NULL
Brian	Denver	38

If you ask for a column the table doesn't have, the query won't run. The message is no such column, with the name SQL couldn't find.

This error does point at the exact word. It's almost always one letter too many or a column from another table.

Exercises in this lesson

You do them in the app, which checks them on the spot and explains why.

1. Complete the code
Which cities does the store send packages to? Fill in the blank to see only that.
```
SELECT ___
FROM customers;
```
2. Multiple choice
The store has 8 products and someone asks for only the prices. What does this query return?
```
SELECT price
FROM products;
```
3. Complete the code
In the warehouse, every box is marked with its product's number. Fill in the blank to see that number next to the name.
```
SELECT id___ name
FROM products;
```
4. Put the lines in order
The store wants to know how old its customers are. Put the query in order to see each one's name and age. Two lines don't belong.
5. Find the bug
The shelf signs need each product's name and category. SQL complains about the word FROM. Which line is the real error on?
```
SELECT name, category,
FROM products;
```
6. Multiple choice
In the products table, the name column comes before stock. In this query's result, which column ends up on the left?
```
SELECT stock, name
FROM products;
```
7. Complete the code
The warehouse sorts outgoing packages by city, so its list reads better with the city first and the name after. Fill in the blanks.
```
SELECT ___, ___
FROM customers;
```
8. Multiple choice
The store's weekly report uses SELECT * FROM customers;. One day someone adds a phone column to the table. What happens to the report?
9. Put the lines in order
The catalog lists each product's name, category, and price, in that order. Put the query in order, with one column per line. One line doesn't belong.
10. Find the bug
Someone wants to compare prices by section, with each product's category and price. Which line has the error?
```
SELECT category,
  prices
FROM products;
```
11. Write the code
The shelf is being rearranged by section. Write a query that shows two columns for each product, first its category and then its name.
```
SELECT
FROM products;
```

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