Create readme.md

Author: elmoallistair

Introduction to Relational Databases in SQL/01 Your first database/01 Attributes of relational databases/readme.md

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+## Attributes of relational databases
+
+In the video, we talked about some basic facts about relational databases. Which of the following statements does not hold true for databases? Relational databases ...
+
+**Possible Answers**
+
+* ... store different real-world entities in different tables.
+* ... allow to establish relationships between entities.
+* ... are called "relational" because they store data only about people.
+* ... use constraints, keys and referential integrity in order to assure data quality.
+
+**Answer**
+
+> ... are called "relational" because they store data only about people.

Introduction to Relational Databases in SQL/02 Enforce data consistency with attribute constraints/03 Type CASTs/query.sql

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+-- Calculate the net amount as amount + fee
+-- SELECT transaction_date, amount + fee AS net_amount
+SELECT transaction_date, amount + CAST(fee AS integer) AS net_amount
+FROM transactions;

Introduction to Relational Databases in SQL/02 Enforce data consistency with attribute constraints/07 What happens if you try to enter NULLs/readme.md

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+## What happens if you try to enter NULLs?
+
+Execute the following statement:
+
+```
+INSERT INTO professors (firstname, lastname, university_shortname)
+VALUES (NULL, 'Miller', 'ETH');
+```
+
+<hr>
+
+Why does this throw an error?
+
+**Possible Answers**
+
+* Professors without first names do not exist.
+* Because a database constraint is violated.
+* Error? This works just fine.
+* `NULL` is not put in quotes.
+
+**Answer**
+
+> Because a database constraint is violated.

Introduction to Relational Databases in SQL/03 Uniquely identify records with key constraints/03 Identify the primary key/readme.md

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+## Identify the primary key
+
+Have a look at the example table from the previous video. As the database designer, you have to make a wise choice as to which column should be the primary key.
+
+```
+ license_no | serial_no | make | model | year
+--------------------+-----------+------------+---------+------
+ Texas ABC-739 | A69352 | Ford | Mustang | 2
+ Florida TVP-347 | B43696 | Oldsmobile | Cutlass | 5
+ New York MPO-22 | X83554 | Oldsmobile | Delta | 1
+ California 432-TFY | C43742 | Mercedes | 190-D | 99
+ California RSK-629 | Y82935 | Toyota | Camry | 4
+ Texas RSK-629 | U028365 | Jaguar | XJS | 4
+```
+
+<hr>
+
+Which of the following column or column combinations could best serve as primary key?
+
+**Possible Answers**
+
+* PK = {make}
+* PK = {model, year}
+* PK = {license_no}
+* PK = {year, make}
+
+**Answer**
+
+> PK = {license_no}

Introduction to Relational Databases in SQL/04 Glue together tables with foreign keys/07 Referential integrity violations/readme.md

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+## Referential integrity violations
+
+Given the current state of your database, what happens if you execute the following SQL statement?
+
+```
+DELETE FROM universities WHERE id = 'EPF';
+```
+
+**Possible Answers**
+
+* It throws an error because the university with ID "EPF" does not exist.
+* The university with ID "EPF" is deleted.
+* It fails because referential integrity from `universities` to `professors` is violated.
+* It fails because referential integrity from `professors` to `universities` is violated.
+
+**Answer**
+
+> It fails because referential integrity from `professors` to `universities` is violated.