SQL 十四分鐘速成班！沒錯不要懷疑，資料庫語法比中午決定吃什麼還要簡單！

PAPAYA 電腦教室

28 Sept 202413:36

Summary

TLDRThis video provides an introduction to SQL, a crucial language for interacting with databases. Using the Sqliteviz website for hands-on practice, viewers learn to query, update, and manage data without software installation. Key SQL concepts such as SELECT, WHERE, JOIN, and functions like AVG, SUM, and COUNT are covered, along with tips on improving code readability and performance. The tutorial also includes creating tables, inserting data, and performing cross-table queries, making it a comprehensive guide for beginners looking to enhance their data management skills.

Takeaways

💡 SQL is a critical language for interacting with databases, allowing users to query, update, and manage data efficiently.
🌐 SQLiteViz is a web-based platform where beginners can practice SQL without needing to install any software.
📋 You can load data into SQLiteViz by dragging a file into the interface, and it automatically creates a table based on the file's content.
🔍 The SELECT statement is used to retrieve data, where '*' selects all columns, or you can specify columns by name.
📊 The LIMIT and OFFSET clauses are useful for controlling the number of rows returned and for pagination, especially when dealing with large datasets.
🔎 The WHERE clause is essential for filtering data based on conditions, and ORDER BY is used for sorting the results.
✨ Wildcard characters like '%' and '_' can be used with the LIKE keyword for flexible text matching in SQL queries.
🧮 SQL functions such as AVG, SUM, MAX, MIN, and COUNT help in aggregating data, and aliasing columns improves readability.
📊 GROUP BY and HAVING allow users to group data and filter aggregated results, respectively, providing more structured data analysis.
🔗 SQL JOINs, such as LEFT JOIN and INNER JOIN, enable combining data from multiple tables, making cross-table queries possible.

Q & A

What is SQL used for?
-SQL is used for interacting with databases, including querying, adding, or updating data. It is a key skill for anyone involved in data analysis and decision-making.
What are some popular database management systems that use SQL?
-Some popular database management systems that use SQL include MySQL, PostgreSQL, Microsoft SQL Server, and Oracle.
What is Sqliteviz, and why is it useful for beginners?
-Sqliteviz is an online tool that allows users to practice SQL directly in a browser without installing any software. It's intuitive and beginner-friendly, making it easy to learn SQL.
What does the SELECT statement do in SQL?
-The SELECT statement in SQL is used to retrieve data from a database. It can return all columns (using *) or specific ones from a table.
What is the purpose of the LIMIT and OFFSET clauses in SQL?
-The LIMIT clause restricts the number of results returned by a query, while OFFSET allows you to skip a certain number of rows, enabling pagination of large datasets.
How do you filter results in SQL using the WHERE clause?
-The WHERE clause is used to set conditions for filtering the results, such as showing only data that meets specific criteria, like rows with a specific value in a column.
How does the ORDER BY clause function in SQL?
-The ORDER BY clause is used to sort the query results based on one or more columns, either in ascending (ASC) or descending (DESC) order.
What are SQL aggregate functions, and can you give examples?
-SQL aggregate functions perform calculations on multiple rows of data. Examples include AVG (average), SUM (sum), MAX (maximum), MIN (minimum), and COUNT (row count).
What is the difference between GROUP BY and HAVING clauses in SQL?
-GROUP BY groups rows that have the same values in specified columns, while HAVING is used to filter groups based on a condition after aggregation.
How can you join tables in SQL, and what is the difference between LEFT JOIN and INNER JOIN?
-SQL joins are used to combine rows from two or more tables based on a related column. LEFT JOIN returns all records from the left table and matching records from the right table, while INNER JOIN only returns records that have matching values in both tables.

Outlines

00:00

💻 Introduction to SQL and Sqliteviz

This section introduces SQL as a crucial programming language used for interacting with databases, such as querying, adding, and updating data. The speaker focuses on Sqliteviz, a website that allows users to practice SQL directly in a browser, making it accessible for beginners. A sample student data file is provided for the tutorial, and the speaker demonstrates how to import it into Sqliteviz. The section also explains the syntax of basic SQL commands like SELECT, including how to fetch all or specific columns from a table, and the importance of using comments and semicolons for clarity and code completeness.

05:00

🔍 Filtering Data with WHERE and LIMIT Clauses

This paragraph expands on how SQL can filter data using the WHERE clause. Examples include filtering by specific classes, sorting results using ORDER BY, and limiting the number of rows returned using the LIMIT clause. The OFFSET clause is also introduced to paginate results, allowing for more control when working with large datasets. The speaker emphasizes best practices for writing readable SQL code, such as separating clauses onto different lines.

10:01

🔢 Using Logical Operators and Pattern Matching in SQL

Here, the speaker demonstrates how SQL can use logical operators like AND and OR to perform more complex filtering, such as selecting students within specific score ranges. Additionally, pattern matching with LIKE and wildcard characters is explained, allowing for flexible text-based queries. The speaker also highlights the importance of parentheses to control operator precedence and introduces the IN keyword for simplifying OR conditions. The section concludes by discussing functions like AVG, SUM, MAX, and COUNT for data aggregation.

📊 GROUP BY and HAVING for Aggregating Data

This section focuses on aggregating data using GROUP BY to calculate averages or sums for different categories (e.g., class averages). The HAVING clause is introduced to filter grouped data, and the speaker emphasizes the importance of correct clause order in SQL statements to avoid errors. Examples include filtering classes with an average score above a certain threshold and applying ROUND to round numbers to a specific decimal place for better readability.

🛠 Creating and Modifying Tables in SQL

The speaker explains how to create new tables in SQL with the CREATE TABLE command, defining columns and specifying data types such as INT and VARCHAR. The importance of setting a primary key to ensure unique and non-null values is emphasized. They demonstrate inserting data into the table with the INSERT INTO command, updating records with the UPDATE command, and safely deleting records or entire tables using DELETE and DROP TABLE. The speaker highlights common pitfalls, such as forgetting WHERE clauses when updating or deleting, which could result in unintended changes.

🔗 Joining Tables for Complex Queries

The final section introduces SQL table joins, focusing on LEFT JOIN to combine data from two tables (students and clubs). It explains how to specify the relationship between tables using shared columns (e.g., club IDs) and describes how LEFT JOIN will return all rows from the left table, even if there’s no match in the right table. INNER JOIN is also discussed for cases where only matched rows are needed. The section concludes by encouraging further practice with JOINs and summarizing the SQL concepts covered in the video.

Mindmap

Keywords

💡SQL

SQL, or Structured Query Language, is the programming language used to interact with databases, allowing users to perform tasks such as querying, inserting, or updating data. In the video, SQL is presented as a core skill for data analysis and decision-making, with practical examples shown using a student database. The video emphasizes how SQL is used for retrieving specific information from databases.

💡SQLiteviz

SQLiteviz is a web-based tool that allows users to practice SQL queries directly in their browser without needing to install software. The video demonstrates how this tool is particularly intuitive and user-friendly for beginners, allowing them to easily practice SQL queries on a sample student dataset by uploading a file.

💡SELECT statement

The SELECT statement is a fundamental SQL command used to retrieve data from a database. In the video, it is illustrated as a query that selects all columns from the 'students' table by using the asterisk (*) symbol. The example showcases how SELECT works to return data from specific fields such as student names, classes, and grades.

💡WHERE clause

The WHERE clause is an SQL keyword used to filter results based on specific conditions. The video explains how this clause is used to limit the data retrieved, such as filtering students by class or grade. It is also highlighted that WHERE can be combined with other operators, such as ‘AND’ or ‘OR’, to refine searches further.

💡ORDER BY

ORDER BY is an SQL clause used to sort query results by one or more columns in ascending or descending order. The video illustrates this by showing how student records can be sorted by class or exam scores, either in ascending (lowest to highest) or descending (highest to lowest) order.

💡JOIN

JOIN is a keyword used in SQL to combine rows from two or more tables based on a related column. In the video, the LEFT JOIN is demonstrated by linking student data with their corresponding club names from another table. The concept of INNER JOIN is also introduced, where only records with matches in both tables are returned.

💡GROUP BY

GROUP BY is an SQL clause used to group rows that have the same values in specified columns, often used with aggregate functions like COUNT, AVG, or SUM. In the video, it is used to show how to calculate the average score of students within each class, grouping the data by the 'class' column.

💡HAVING clause

The HAVING clause is similar to WHERE, but it is used to filter the results after the data has been grouped using the GROUP BY clause. In the video, it is shown to filter out groups (e.g., classes) where the average score is less than 80, demonstrating its role in refining grouped results.

💡LIMIT and OFFSET

LIMIT is used to restrict the number of rows returned by a query, while OFFSET skips a specified number of rows before starting to return rows. In the video, these clauses are demonstrated when working with larger datasets to avoid retrieving all data at once, which can be inefficient. For example, LIMIT 5 returns only five records, while OFFSET allows skipping the first few records.

💡INSERT INTO

INSERT INTO is the SQL statement used to add new data into a table. The video shows how to use this command to insert club information into a newly created 'clubs' table. The importance of ensuring that the data matches the columns of the table is emphasized, along with the role of VALUES to input actual data.

Highlights

Introduction to SQL as a language for interacting with databases, essential for data analysis and decision-making.

Common databases where SQL is used include MySQL, PostgreSQL, Microsoft SQL Server, and Oracle.

Demonstration using the Sqliteviz website to practice SQL without installing software, ideal for beginners.

Explanation of basic SQL syntax, such as SELECT, FROM, and how to retrieve all columns from a table.

Clarification on using or omitting double quotes around table names depending on the database system.

Introduction to using LIMIT and OFFSET in SQL queries to control the number of returned rows.

Overview of WHERE clause for filtering data based on specific conditions, such as class or exam scores.

Explanation of ORDER BY for sorting data, including ascending and descending order for columns like scores.

Introduction to wildcard characters like % and _ for flexible text matching using the LIKE keyword.

Explanation of AND, OR, and parentheses to control logical precedence in complex queries.

Introduction to aggregate functions such as AVG, SUM, MAX, MIN, and COUNT for summarizing data.

Overview of GROUP BY for calculating grouped aggregates, and HAVING for filtering grouped data.

Using DISTINCT to retrieve unique values from a table, and handling NULL values with WHERE.

Creating and managing new tables using CREATE TABLE, specifying data types and primary keys.

Performing joins between tables, such as using LEFT JOIN and INNER JOIN to combine related data from different tables.