1. SQL – Where

2. Where SELECT ... FROM ... WHERE ...;
The where clause stipulates a test (predicate) that each row must pass to be returned in the select statement.
SELECT * FROM students WHERE spell_performance > 8;

3. SELECT * FROM students WHERE spell_performance > 8;
first_name
spell_performance
type_cat
Michael 8 No
Austin
cat
Adi 10
Dog
Daniel 9
Wesley

4. SELECT * FROM students WHERE spell_performance > 8;
first_name
spell_performance
type_cat
Michael 8 No
Austin
cat
Adi 10
Dog
Daniel 9
Wesley

5. SELECT * FROM students WHERE spell_performance > 8;
first_name
spell_performance
type_cat
Adi 10
Dog
Daniel 9
cat

6. What should happen for the query:
SELECT * FROM students WHERE spell_performance > 10;
first_name
spell_
performance
type_cat
Michael 8 No
Austin
cat
Adi 10
Dog
Daniel 9
Josh
NULL
It should return 0 rows
It should raise a syntax error
It should return 1 row
It should raise a "no rows" error

7. SQL operators Arithmetic Operators: Comparison Operators:
+ - * / %
Comparison Operators:
= != <> > < >= <=
Logical Operators
ALL AND ANY BETWEEN EXISTS IN LIKE NOT OR IS

8. AND AND OR first_name spell_performance type_cat Michael 8 No Austin
Adi 10
Dog
Daniel 9
Wesley
SELECT * FROM students WHERE spell_performance > 8 AND type_cat = 'cat';
1 row
SELECT * FROM students WHERE spell_performance > 8 OR type_cat = 'cat';
4 rows (poor Michael)

9. Between first_name spell_performance type_cat Michael 8 No Austin cat
Adi 10
Dog
Daniel 9
Wesley
SELECT * FROM students WHERE spell_performance BETWEEN 9 AND 10;
2 rows
Between is inclusive of boundaries
SELECT * FROM students WHERE spell_performance NOT BETWEEN 9 AND 10;
3 rows

10. IS first_name spell_performance fav_color Michael 8 NULL Austin 'red'
Adi 10
Daniel 9
'Green'
Wesley
'dark green'
SELECT * FROM students WHERE fav_color IS NULL;
2 rows
SELECT * FROM students WHERE fav_color IS NOT NULL;
3 rows
The "IS" operator is only to be used to test for NULL values.

11. Like first_name spell_performance fav_color Michael 8 NULL Austin
'red'
Adi 10
Daniel 9
'Green'
Wesley
'dark green'
SELECT * FROM students WHERE fav_color LIKE '%reen';
2 rows
% is a wildcard that can match 0 or more characters.
Should have been *, but it was already taken
_ (underscore) is a wildcard representing a single character
Should have been . grrrrr

12. IN first_name spell_performance fav_color Michael 8 NULL Austin 'red'
Adi 10
Daniel 9
'Green'
Wesley
'dark green'
SELECT * FROM students WHERE spell_performance IN (5, 8, 10);
4 rows
IN tests for inclusion in a static, parenthesized list.

13. EXISTS, ANY, ALL
These operators use sub-queries, which we'll get to later in the course.

14. Comparing NULLs to values
The logic of conditions in SQL is really 3-valued logic: TRUE, FALSE, UNKNOWN.
Comparing any value (including NULL itself) with NULL yields UNKNOWN.
A tuple is in a query answer if and only if the WHERE clause is TRUE (not FALSE or UNKNOWN).

15. What should happen for the query:
first_name
spell_
performance
type_cat
Michael 8 No
Austin
cat
Adi 10
Dog
Daniel 9
Wesley
Josh
NULL
SELECT * FROM students WHERE NOT spell_performance > 8;
It should return 0 rows
It should return 1 row
It should return 2 rows
It should return 3 rows

16. Three-Valued Logic
To understand how AND, OR, and NOT work in 3-valued logic, think of TRUE = 1, FALSE = 0, and UNKNOWN = ½.
AND = MIN; OR = MAX, NOT(x) = 1-x.
Example:
TRUE AND (FALSE OR NOT (UNKNOWN)) = MIN(1, MAX(0, (1 - ½ ))) =
MIN(1, MAX(0, ½ )) = MIN(1, ½ ) = ½.

17. Example with null first_name spell_performance Yi NULL
SELECT * FROM students WHERE spell_performance > 5 OR spell_performance <= 5;
UNKNOWN
UNKNOWN
UNKNOWN

18. Reason: 2-Valued Laws != 3-Valued Laws
Some common laws, like commutativity of AND, hold in 3-valued logic.
But not others, e.g., the law of the excluded middle : p OR NOT p = TRUE.
When p = UNKNOWN, the left side is MAX( ½, (1 – ½ )) = ½ != 1.