Add postgres day 1 homework

postgres_homework.md

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+# PostgreSQL homework
+
+## Day 1
+
+1. Select all the tables we created (and only those) from `pg_class`.
+
+ ```
+ SELECT relname FROM pg_class
+ WHERE relname !~ '^(pg_|sql_)'
+ AND relkind = 'r';
+ ```
+
+2. Write a query that finds the country name of the LARP Club event.
+
+ ```
+ SELECT c.country_name FROM countries c
+ JOIN venues v ON c.country_code = v.country_code
+ JOIN events e ON v.venue_id = e.venue_id
+ WHERE e.title = 'LARP Club';
+ ```
+
+3. Alter the `venues` table to contain a boolean column called `active`, with the default value of `true`.
+
+ ```
+ ALTER TABLE venues
+ ADD COLUMN active BOOLEAN DEFAULT TRUE;
+ ```