Hands-On Differential Privacy

Appendix D. Structured Query Language (SQL)

For our purposes, a database is a collection of tables containing data that you can query data from. A table is a collection of data with a known structure, and a query is a statement about what you want from the table.

For example, consider a database of students in a school, where each table is a class (Tables D-1 and D-2).

Table D-1. ComputerScience101

student_id	first_name	last_name	major	grade
58394	Doy	Easterbrook	computer science	80
29485	Merridie	Blockwell	history	85
92554	Denys	Phorsby	physics	95
02359	Coreen	Otley	english	90
02945	Merci	Wiszniewski	history	95

Table D-2. History101

student_id	first_name	last_name	major	grade
09528	Teodoro	Anscombe	english	85
10394	Kala	Tidcombe	english	85
85422	Theodosia	Kelson	computer science	90
25925	Saunderson	Dunlap	english	90
29485	Merridie	Blockwell	history	95
92554	Denys	Phorsby	physics	95
02359	Coreen	Otley	english	85
02945	Merci	Wiszniewski	history	95

To see the student_id for every student in History 101, you would use the following query:

SELECT student_id FROM History101;

and would receive this output:

+------------+
| student_id |
+------------+
| 09528      |
| 10394      |
| 85422      |
| 25925      |
| 29485      |
| 92554      |
| 02359      |
| 02945      |
+------------+

This type of SELECT statement works on any combination of the columns:

SELECT student_id, major FROM History101;

+------------+------------------+ | student_id | major | +------------+------------------+ ...