feat: add postgres cross table row deletions til

postgres/cross-table-row-deletions.md

@@ -0,0 +1,75 @@
+# Cross-Table Row Deletions
+
+*Context*
+Need to `DELETE` all rows from a table satisfying a certain condition based on a value in another table.
+
+*Assumptions*
+Table: `Person`
+`Person` Columns: `person_id`, `city_id`, `email`
+
+Table: `City`
+`City` Columns: `city_id`, `name`, `population`
+
+```sql
+CREATE TABLE City (
+    city_id INT PRIMARY KEY,
+    name TEXT,
+    population INT
+);
+
+CREATE TABLE Person (
+    person_id INT PRIMARY KEY,
+    email TEXT,
+    city_id INT,
+    FOREIGN KEY (city_id) REFERENCES City(city_id)
+);
+
+INSERT INTO
+    City (city_id, name, population)
+VALUES 
+    (1, 'New York', 8419000), 
+    (2, 'Los Angeles', 3971000), 
+    (3, 'Chicago', 2716000), 
+    (4, 'Houston', 2325500), 
+    (5, 'Phoenix', 1680000);
+
+INSERT INTO
+    Person (person_id, email, city_id)
+VALUES
+    (1, '[email protected]', 1),
+    (2, '[email protected]', 2),
+    (3, '[email protected]', 3),
+    (4, '[email protected]', 4),
+    (5, '[email protected]', 5);
+```
+
+*Goal*
+Delete all people from cities with a population of less than 1,000,000 people.
+
+*Solution*
+Standard-compliant solution:
+
+```sql
+DELETE FROM Person
+WHERE
+    city_id IN (
+        SELECT
+            city_id
+        FROM
+            City
+        WHERE
+            population < 1000000
+    );
+```
+
+`USING` (Extension)[^1] solution:
+
+```sql
+DELETE FROM Person USING City
+WHERE
+    Person.city_id = City.city_id
+    AND City.population < 1000000;
+```
+
+[^1]: [This command does not conform to the SQL standard](https://www.postgresql.org/docs/current/sql-delete.html). `USING` and `RETURNING` clauses
+are postgres exentions.