SQL
What do SQL constraints do?
Tells a database to reject inserted data that doesn't follow certain restrictions.
PRIMARY _KEY
[Constraint] Rejects any new duplicate value that is already in a selected column.
One PRIMARY KEY per database.
UNIQUE
[Constraint] Same as PRIMARY KEY but no limit on how many columns per database.
NOT NULL
[Constraint] Sets a column to require a value, otherwise value will not be stored.
DEFAULT
[Constraint] Adds placeholder text if no value is submitted
What's a Relational Database?
A database that organizes data into one or more tables.
What's a Table?
A collection of data organized into rows and columns.
What do SQL clauses do?
Causes are commands used to manipulate a database.
What are Aggregates?
Calculations performed on multiple rows of a table.
What are five essential aggregates?
- COUNT()
- SUM()
- MAX() / MIN()
- AVG()
- ROUND()
COUNT()
Returns the number of rows that match the specified criteria.
COUNT()
Returns the number of rows that match the specified criteria.
SUM()
Returns the sum of all the values in a column.
MAX()
Returns the largest value in a column.
MIN()
Returns the smallest value in a column.
AVG()
Returns the average value in a column.
ROUND()
Rounds a value to the nearest integer.
Can also round to a specific number of decimals if a value is provided.