Your first query

SQL · Unit 1: First queries

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SQL
10 exercises

Who are this store's customers? The answer is stored in a table named customers, and this line asks for it. Each row is a person and each column is one fact about them.

See the NULL in Chloë's city? It's not a city: it's how the table marks a value it doesn't have.

SELECT * FROM customers;

Returns

id	name	city	age
1	Ann	Denver	34
2	Luke	Austin	27
3	Chloë	NULL	41
4	Mason	Tampa	19
5	Valerie	Denver	52
6	Dylan	Boston	27
7	Hannah	Austin	NULL
8	Brian	Denver	38

That line is a query, and it reads almost like a sentence. SELECT says what you want to see, and the * means every column.

FROM says which table. And what does the store sell? Same query, different table.

SELECT * FROM products;

Returns

id	name	category	price	stock
1	Keyboard	Accessories	45.5	12
2	Mouse	Accessories	19.9	30
3	Monitor	Displays	189	5
4	Cable	Accessories	5.25	0
5	Laptop	Devices	899	3
6	Tablet	Devices	320	7
7	Headphones	Audio	59.9	15
8	Speaker	Audio	74	0

SQL doesn't care about line breaks, which is why it needs the ; to know where a query ends.

You do care. With SELECT on one line and FROM on another it's easier to read, and when something breaks you'll know which line to check.

SELECT *
FROM customers;

Returns

id	name	city	age
1	Ann	Denver	34
2	Luke	Austin	27
3	Chloë	NULL	41
4	Mason	Tampa	19
5	Valerie	Denver	52
6	Dylan	Boston	27
7	Hannah	Austin	NULL
8	Brian	Denver	38

When a query is written wrong, SQL stops and tells you the word where it got lost.

That word is almost never the mistake: it's the first one that no longer fit. Your mistake is usually right before it, so read the message and then look back.

SQL is about fifty years old, and almost every database speaks it: MySQL, PostgreSQL, SQLite, and many more.

Here your queries run on SQLite, inside your phone. Each database has its quirks, and we'll tell you when something changes from one to another.

Exercises in this lesson

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

1. Multiple choice
In the customers table, what is a row?
2. Multiple choice
Tomorrow a new customer, Emily, comes in and the store adds her. What does the customers table gain?
3. Complete the code
Someone new at the store wants to see everything it knows about each customer. Fill in the blank.
```
SELECT ___ FROM customers;
```
4. Complete the code
The store also records every purchase, in the orders table. Fill in the blank to see it.
```
SELECT * ___ orders;
```
5. Put the lines in order
What does the store sell? Put in order the query that answers it. One line doesn't belong.
6. Multiple choice
Which query is written correctly?
7. Find the bug
Here are two queries, one to see who buys and one to see what's for sale. SQL complains about the second SELECT. Which line is the real error on?
```
SELECT *
FROM customers
SELECT *
FROM products;
```
8. Put the lines in order
Put the two queries in order to see first who buys and then what each person has ordered. One line doesn't belong.
9. Multiple choice
You're learning SQL here, with SQLite. Tomorrow a job has you using PostgreSQL. Will what you know still help?
10. Write the code
The store wants to go over everything it sells. Write the query that shows the whole products table, with all its columns.
```
SELECT
```

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