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Questions and Answers
What is the primary goal of data normalization?
What is the primary goal of data normalization?
To minimize data redundancy and dependency
What is the difference between an INNER JOIN and a LEFT JOIN?
What is the difference between an INNER JOIN and a LEFT JOIN?
An INNER JOIN returns only the rows that have matching values in both tables, while a LEFT JOIN returns all the rows from the left table and the matching rows from the right table.
What is the purpose of the PRIMARY KEY constraint in a CREATE TABLE statement?
What is the purpose of the PRIMARY KEY constraint in a CREATE TABLE statement?
To create a unique identifier for each row
What is indexing in the context of query optimization?
What is indexing in the context of query optimization?
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What is the purpose of a scalar subquery?
What is the purpose of a scalar subquery?
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What is denormalization in the context of query optimization?
What is denormalization in the context of query optimization?
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What is the purpose of the EXPLAIN tool in query optimization?
What is the purpose of the EXPLAIN tool in query optimization?
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What is the difference between a table subquery and a row subquery?
What is the difference between a table subquery and a row subquery?
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What is the purpose of the SELECT command in SQL?
What is the purpose of the SELECT command in SQL?
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What is the difference between single quotes and double quotes in SQL string literals?
What is the difference between single quotes and double quotes in SQL string literals?
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What is the function of the WHERE clause in a SQL query?
What is the function of the WHERE clause in a SQL query?
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What is the purpose of the IN operator in a SQL query?
What is the purpose of the IN operator in a SQL query?
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What does the SUM aggregate function do in SQL?
What does the SUM aggregate function do in SQL?
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What is the main difference between the AND and OR operators in a SQL WHERE clause?
What is the main difference between the AND and OR operators in a SQL WHERE clause?
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What is the purpose of the CREATE command in SQL?
What is the purpose of the CREATE command in SQL?
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What does the UPDATE command do in SQL?
What does the UPDATE command do in SQL?
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What is the purpose of the MAX function in SQL, and provide an example of its usage?
What is the purpose of the MAX function in SQL, and provide an example of its usage?
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What is the difference between an INNER JOIN and a LEFT JOIN, and provide an example of each?
What is the difference between an INNER JOIN and a LEFT JOIN, and provide an example of each?
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What is the purpose of the COUNT function in SQL, and provide an example of its usage?
What is the purpose of the COUNT function in SQL, and provide an example of its usage?
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What is a subquery in SQL, and provide an example of its usage?
What is a subquery in SQL, and provide an example of its usage?
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How does the RIGHT JOIN differ from the LEFT JOIN in SQL, and provide an example of its usage?
How does the RIGHT JOIN differ from the LEFT JOIN in SQL, and provide an example of its usage?
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What is the purpose of using aggregate functions like AVG, MAX, and MIN in SQL, and provide an example of each?
What is the purpose of using aggregate functions like AVG, MAX, and MIN in SQL, and provide an example of each?
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Study Notes
Data Normalization
- Goal: To minimize data redundancy and dependency
- Normalization rules:
- First Normal Form (1NF): Each table cell must contain a single value
- Second Normal Form (2NF): Each non-prime attribute in a table must depend on the entire primary key
- Third Normal Form (3NF): If a table is in 2NF, and a non-prime attribute depends on another non-prime attribute, then it should be moved to a separate table
- Boyce-Codd Normal Form (BCNF): A table is in 3NF and there are no transitive dependencies
- Higher Normal Forms: 4NF, 5NF, and higher, which deal with more complex dependencies
Joining Tables
- Types of joins:
- INNER JOIN: Returns only the rows that have matching values in both tables
- LEFT JOIN: Returns all the rows from the left table, and the matching rows from the right table
- RIGHT JOIN: Returns all the rows from the right table, and the matching rows from the left table
- FULL JOIN: Returns all the rows from both tables
- CROSS JOIN: Returns the Cartesian product of both tables
- Join syntax:
-
SELECT * FROM table1 INNER JOIN table2 ON table1.column = table2.column -
SELECT * FROM table1 LEFT JOIN table2 ON table1.column = table2.column
-
Create Table Syntax
- Basic syntax:
CREATE TABLE table_name (column1 data_type, column2 data_type, ...) - Data types:
- INT: Whole numbers
- VARCHAR: Character strings with a maximum length
- DATE: Dates in the format 'YYYY-MM-DD'
- TIME: Times in the format 'HH:MM:SS'
- Constraints:
- PRIMARY KEY: Unique identifier for each row
- UNIQUE: Ensures all values in a column are unique
- NOT NULL: Ensures a column cannot be null
- CHECK: Ensures a column meets a specific condition
Query Optimization
- Techniques:
- Indexing: Creating indexes on frequently used columns to speed up queries
- Caching: Storing frequently accessed data in memory for faster retrieval
- Optimizing SQL: Rewriting queries to minimize the number of rows and columns accessed
- Denormalization: Storing redundant data to reduce the number of joins
- Tools:
- EXPLAIN: Analyzing the query plan to identify bottlenecks
- Query profiling: Measuring the execution time and resource usage of queries
Subqueries
- A query nested inside another query
- Types:
- Scalar subquery: Returns a single value
- Row subquery: Returns a single row
- Table subquery: Returns a table
- Syntax:
-
SELECT * FROM table WHERE column = (SELECT value FROM table2) -
SELECT * FROM table WHERE column IN (SELECT value FROM table2) -
SELECT * FROM table WHERE EXISTS (SELECT * FROM table2 WHERE condition)
-
Data Normalization
- Minimizes data redundancy and dependency
- Normalization rules:
- First Normal Form (1NF): Each table cell must contain a single value
- Second Normal Form (2NF): Each non-prime attribute in a table must depend on the entire primary key
- Third Normal Form (3NF): If a table is in 2NF, and a non-prime attribute depends on another non-prime attribute, then it should be moved to a separate table
- Boyce-Codd Normal Form (BCNF): A table is in 3NF and there are no transitive dependencies
- Higher Normal Forms: 4NF, 5NF, and higher, which deal with more complex dependencies
Joining Tables
- Types of joins:
- INNER JOIN: Returns only the rows that have matching values in both tables
- LEFT JOIN: Returns all the rows from the left table, and the matching rows from the right table
- RIGHT JOIN: Returns all the rows from the right table, and the matching rows from the left table
- FULL JOIN: Returns all the rows from both tables
- CROSS JOIN: Returns the Cartesian product of both tables
- Join syntax:
- SELECT * FROM table1 INNER JOIN table2 ON table1.column = table2.column
- SELECT * FROM table1 LEFT JOIN table2 ON table1.column = table2.column
Creating Tables
- Basic syntax: CREATE TABLE table_name (column1 data_type, column2 data_type,...)
- Data types:
- INT: Whole numbers
- VARCHAR: Character strings with a maximum length
- DATE: Dates in the format 'YYYY-MM-DD'
- TIME: Times in the format 'HH:MM:SS'
- Constraints:
- PRIMARY KEY: Unique identifier for each row
- UNIQUE: Ensures all values in a column are unique
- NOT NULL: Ensures a column cannot be null
- CHECK: Ensures a column meets a specific condition
Query Optimization
- Techniques:
- Indexing: Creating indexes on frequently used columns to speed up queries
- Caching: Storing frequently accessed data in memory for faster retrieval
- Optimizing SQL: Rewriting queries to minimize the number of rows and columns accessed
- Denormalization: Storing redundant data to reduce the number of joins
- Tools:
- EXPLAIN: Analyzing the query plan to identify bottlenecks
- Query profiling: Measuring the execution time and resource usage of queries
Subqueries
- A query nested inside another query
- Types:
- Scalar subquery: Returns a single value
- Row subquery: Returns a single row
- Table subquery: Returns a table
- Syntax:
- SELECT * FROM table WHERE column = (SELECT value FROM table2)
- SELECT * FROM table WHERE column IN (SELECT value FROM table2)
- SELECT * FROM table WHERE EXISTS (SELECT * FROM table2 WHERE condition)
Basics of SQL
- SQL is a standard language for managing relational databases, used to store, manipulate, and retrieve data stored in a database.
- SQL is not case-sensitive, but it's common to write commands in uppercase and table/column names in lowercase.
Data Types in SQL
- Integer: whole numbers, e.g. 1, 2, 3, etc.
- String: character strings, e.g. 'hello', "hello", etc. Strings can be enclosed in single quotes or double quotes.
- Date: dates, e.g. '2022-01-01', etc.
- Boolean: true or false values.
Basic SQL Commands
- SELECT: retrieves data from a database table, e.g.
SELECT * FROM customers; - CREATE: creates a new database table or other object, e.g.
CREATE TABLE customers (id INT, name VARCHAR(255), email VARCHAR(255)); - INSERT: inserts new data into a database table, e.g.
INSERT INTO customers (id, name, email) VALUES (1, 'John Doe', '[email protected]'); - UPDATE: updates existing data in a database table, e.g.
UPDATE customers SET name = 'Jane Doe' WHERE id = 1; - DELETE: deletes data from a database table, e.g.
DELETE FROM customers WHERE id = 1;
SQL Query Clauses
- WHERE: specifies conditions for which records to include in the result set, e.g.
SELECT * FROM customers WHERE country='USA'; - AND: combines multiple conditions in a WHERE clause, e.g.
SELECT * FROM customers WHERE country='USA' AND city='New York'; - OR: combines multiple conditions in a WHERE clause, e.g.
SELECT * FROM customers WHERE country='USA' OR country='Canada'; - IN: specifies a list of values to include in the result set, e.g.
SELECT * FROM customers WHERE country IN ('USA', 'Canada', 'Mexico');
Aggregate Functions in SQL
- SUM: calculates the total value of a column, e.g.
SELECT SUM(price) FROM orders; - AVG: calculates the average value of a column, e.g.
SELECT AVG(price) FROM orders; - MAX: returns the maximum value of a column, e.g.
SELECT MAX(price) FROM orders; - MIN: returns the minimum value of a column, e.g.
SELECT MIN(price) FROM orders; - COUNT: returns the number of rows in a table, e.g.
SELECT COUNT(*) FROM customers;
SQL Joins
- INNER JOIN: returns records that have matching values in both tables, e.g.
SELECT * FROM customers INNER JOIN orders ON customers.id = orders.customer_id; - LEFT JOIN: returns all records from the left table, and the matched records from the right table, e.g.
SELECT * FROM customers LEFT JOIN orders ON customers.id = orders.customer_id; - RIGHT JOIN: returns all records from the right table, and the matched records from the left table, e.g.
SELECT * FROM customers RIGHT JOIN orders ON customers.id = orders.customer_id;
SQL Subqueries
- A subquery is a query nested inside another query.
- Subqueries can be used to:
- Return a set of records to use in the main query.
- Perform calculations and return a single value.
- Test for existence or non-existence of records.
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Description
Learn about the rules of data normalization, including 1NF, 2NF, 3NF, and BCNF, to minimize data redundancy and dependency.
