WGU c170 The Most Important SQL Commands(SQL study guide)

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WGU c170 The Most Important SQL Commands(SQL study guide) Notes for C170 Some of The Most Important SQL Commands SELECT - extracts data from a database UPDATE - updates data in a database DELETE ... - deletes data from a database INSERT INTO - inserts new data into a database CREATE DATABASE - creates a new database ALTER DATABASE - modifies a database CREATE TABLE - creates a new table ALTER TABLE - modifies a table DROP TABLE - deletes a table CREATE INDEX - creates an index (search key) DROP INDEX - deletes an index '*' select all the columns Example: SELECT * FROM Customers; 'FROM' pulling info from database 'DISTINCT' = Different Example: SELECT DISTINCT or SELECT FROM Note: The WHERE clause is not only used in SELECT statements, it is also used in UPDATE, DELETE, etc.! ***GOOD EXAMPLES:** SELECT * FROM Customers WHERE City = 'Berlin' OR City = 'London'; SELECT * FROM Customers WHERE Country='Germany' OR Country='Spain'; SELECT * FROM Customers WHERE Country='Germany' AND (City='Berlin' OR City='München'); SELECT * FROM Customers WHERE NOT Country='Germany' AND NOT Country='USA'; SELECT * FROM Customers ORDER BY Country DESC; SELECT * FROM Customers ORDER BY Country, CustomerName; SELECT * FROM Customers ORDER BY Country ASC, CustomerName DESC; INSERT INTO Customers (CustomerName, ContactName, Address, City, PostalCode, Country) VALUES ('Cardinal', 'Tom B. Erichsen', 'Skagen 21', 'Stavanger', '4006', 'Norway'); INSERT INTO Customers (CustomerName, City, Country) VALUES ('Cardinal', 'Stavanger', 'Norway'); SELECT CustomerName, ContactName, Address FROM Customers WHERE Address IS NULL; SELECT CustomerName, ContactName, Address FROM Customers WHERE Address IS NOT NULL; UPDATE Customers SET ContactName = 'Alfred Schmidt', City= 'Frankfurt' WHERE CustomerID = 1; UPDATE Customers SET ContactName='Juan' WHERE Country='Mexico'; Be careful when updating records. If you omit the WHERE clause, ALL records will be updated! UPDATE Customers SET ContactName='Juan'; DELETE FROM Customers WHERE CustomerName='Alfreds Futterkiste'; SELECT TOP 3 * FROM Customers; SELECT * FROM Customers LIMIT 3; SELECT TOP 50 PERCENT * FROM Customers; SELECT TOP 3 * FROM Customers WHERE Country='Germany'; SELECT MIN(Price) AS SmallestPrice FROM Products; SELECT MAX(Price) AS LargestPrice FROM Products; SELECT COUNT(ProductID) FROM Products; SELECT AVG(Price) FROM Products; SELECT SUM(Quantity) FROM OrderDetails; Chapter 1 Key types: Superkey: An attribute or combination of attributes that uniquely identifies each row in a table Candidate key: A minimal (irreducible) superkey; a superkey that does not contain a subset of attributes that is itself a superkey Primary key: A candidate key selected to uniquely identify all other attribute values in any given row; cannot contain null entries Foreign key: An attribute or combination of attributes in one table whose values must either match the primary key in another table or be null Secondary key: An attribute or combination of attributes used strictly for data retrieval purposes Entity Integrity: Requirement: All primary key entries are unique, and no part of a primary key may be null. Purpose: Each row will have a unique identity, and foreign key values can properly reference primary key values. Example: No invoice can have a duplicate number, nor can it be null; in short, all invoices are uniquely identified by their invoice number. Referential Integrity: Requirement: A foreign key may have either a null entry, as long as it is not a part of its table's primary key, or an entry that matches the primary key value in a table to which it is related; (every non-null foreign key value must reference an existing primary key value). Purpose: It is possible for an attribute not to have a corresponding value, but it will be impossible to have an invalid entry; the enforcement of the referential integrity rule makes it impossible to delete a row in one table whose primary key has mandatory matching foreign key values in another table. Example: A customer might not yet have an assigned sales representative (number), but it will be impossible to have an invalid sales representative (number). FK = Foreign key PK = Primary key CHAR = Fixed character length data (1 - 255 characters) VARCHAR = Variable character length data (1 - 2,000 characters) NUMBER = Numeric data. NUMBER (9,2) is used to specify numbers with up to nine digits, including two digits [Show More]

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