CSD123 - Week #2

Review Deck

SQL

Structured Query Language: A declarative language used to talk to Relational Databases.

Declarative Language

You describe WHAT you want (e.g., "Get milk"), not HOW to get it (e.g., "Walk to store").

Order of Execution (Mental Model)

1. FROM (Find Table) -> 2. WHERE (Filter Rows) -> 3. SELECT (Return Columns).

The "Quote" Rule

Numbers are naked (e.g., 50). Text gets dressed in single quotes (e.g., 'Kevin').

SELECT *

Select ALL columns from a table.

WHERE clause

Used to filter rows based on specific conditions.

OR Operator

Logic that EXPANDS results (returns rows matching Condition A OR Condition B).

AND Operator

Logic that RESTRICTS results (returns rows matching BOTH Condition A AND Condition B).

LIKE Operator

Used for pattern matching text (e.g., finding names that start with "B").

% Wildcard

Represents "Zero, One, or Many" characters (e.g., 'B%' finds Bob, Banana, B).

_ Wildcard

Represents "Exactly One" character.

IS NULL

The correct way to check for empty/unknown values (you cannot use = NULL).

ORDER BY default

Ascending (ASC) - A to Z, or Low to High.

DISTINCT

Keyword used to remove duplicate rows from the result set.