adding sql lab excercises for week 6

cst363/lec/lec3.md

@@ -5,20 +5,18 @@
 ### SELECT
 Used to select columns from some table.
 
-> Relational symbol: sigma
+> Relational symbol: pi
 
 ### Projection
 Picks out the rows of a (set of) table(s)
 
-> Relational symbol: pi
+> Relational symbol: sigma
 
 ### Union
 Adds the rows of two tables into some new table. 
 Removes duplicates from resultant table as well.
 
-> `UNION table[table...]`
-
-> Relational symbol: 
+> Relational symbol: U(looks like a U but w i d e) 
 
 ### Relational Algebra on paper
 

cst363/lec/lec6.md

@@ -0,0 +1,18 @@
+# lec6
+
+## Lab activity
+
+This lecture features a lab activity in the lab/ directory named: `courses-ddl.sql` with instructions in `simple-joins-lab.pdf`.
+
+* Note: Just make sure to read int courses-ddl.sql first then courses-small.sql second otherwise there will be random errors.(I'm not taking responsibility for that garbage so don't flame me)
+
+## Natural Joins
+
+`Natural Joins`: allows us to join tables while getting rid of duplicate columns automatically.
+
+Form:
+
+```
+select columns_[...] from tableLeft natural join tableRight
+```
+While ther is no need to write extra `where` statements there is also the issue where there may be accidental matches since attributes are dropped.