SQL as the query language in Intro to Computing - Flowchart & Logic Diagram

Process Overview

SQL (Structured Query Language) is the language used to ask questions and get information from databases. It helps you find, add, change, or remove data by writing simple commands.

Flowchart

Write SQL Query

Yes No↓

Send Query to Database

↓

Database Processes Query

↓

Retrieve Results

↓

Display Results

This flowchart shows the step-by-step process of using SQL: writing a query, checking syntax, sending it to the database, processing, retrieving, and displaying results.

Step-by-Step Trace - 6 Steps

Step 1: Write SQL query to find all students with grade 'A'.

Step 2: Check if the SQL query syntax is correct.

Step 3: Send the query to the database engine.

Step 4: Database processes the query by searching the 'students' table.

Step 5: Retrieve the matching records.

Step 6: Display the results to the user.

Diagram

 +-------------------+       +-------------------+       +-------------------+
 |   User writes     |       |   Database engine  |       |   Data storage    |
 |   SQL query       | ----> |   processes query | ----> |   tables & rows   |
 +-------------------+       +-------------------+       +-------------------+
         ^                                                         |
         |                                                         v
         +-------------------------------------------------<-------+
                          Results returned

This diagram shows how the SQL query flows from the user to the database engine, which processes it by looking into data storage, then returns the results back to the user.

Flowchart Quiz - 3 Questions

What is the first step when using SQL to get data?

A Write the SQL query

B Display results

C Process the query

D Retrieve data

Key Result

SQL works by writing a clear question, checking it, sending it to the database, which then finds and returns the matching data.

Practice

1. What does the SELECT statement do in SQL?

easy

A. It deletes rows from a table.

B. It chooses which columns to show from a table.

C. It creates a new table in the database.

D. It updates values in a table.

Solution

1. Step 1: Understand the purpose of SELECT

   The SELECT statement is used to specify which columns of data you want to see from a table.

2. Step 2: Differentiate from other SQL commands

   Commands like DELETE, CREATE, and UPDATE perform different actions such as removing, creating, or changing data, not selecting columns.

3. Final Answer:

   It chooses which columns to show from a table. -> Option B

4. Quick Check:

   SELECT = choose columns [OK]

Hint: SELECT picks columns to display, not rows or tables [OK]

Common Mistakes:

• Confusing SELECT with DELETE or UPDATE
• Thinking SELECT creates or deletes tables
• Mixing SELECT with WHERE filtering

2. Which of the following is the correct syntax to select the column name from a table called students?

easy

A. SELECT name FROM students;

B. SELECT FROM students name;

C. FROM students SELECT name;

D. SELECT name students FROM;

Solution

1. Step 1: Recall SQL SELECT syntax

   The correct order is SELECT [columns] FROM [table]; so the column name comes after SELECT and the table name after FROM.

2. Step 2: Check each option

   SELECT name FROM students; follows the correct syntax. The other options have incorrect order or missing keywords.

3. Final Answer:

   SELECT name FROM students; -> Option A

4. Quick Check:

   SELECT column FROM table [OK]

Hint: Remember: SELECT columns FROM table; [OK]

Common Mistakes:

• Swapping SELECT and FROM keywords
• Omitting semicolon at the end
• Placing table name before SELECT

3. Given the table employees with columns id, name, and salary, what will this query return?

SELECT name FROM employees WHERE salary > 50000;

medium

A. An error because salary comparison is invalid.

B. All employee names with salary less than 50000.

C. All employee names regardless of salary.

D. All employee names with salary greater than 50000.

Solution

1. Step 1: Understand the WHERE clause

   The WHERE clause filters rows to only those where the salary is greater than 50000.

2. Step 2: Identify the selected column

   The query selects only the name column from the filtered rows.

3. Final Answer:

   All employee names with salary greater than 50000. -> Option D

4. Quick Check:

   WHERE salary > 50000 filters rows [OK]

Hint: WHERE filters rows; SELECT chooses columns [OK]

Common Mistakes:

• Confusing > with < in WHERE clause
• Thinking all columns are returned
• Assuming syntax error due to comparison

4. Identify the error in this SQL query:

SELECT name salary FROM employees;

medium

A. Missing comma between column names.

B. Table name is incorrect.

C. SELECT keyword is misspelled.

D. WHERE clause is missing.

Solution

1. Step 1: Check column list syntax

   When selecting multiple columns, they must be separated by commas. Here, name salary lacks a comma.

2. Step 2: Verify other parts

   The table name employees is correct, SELECT is spelled correctly, and WHERE is optional.

3. Final Answer:

   Missing comma between column names. -> Option A

4. Quick Check:

   Multiple columns need commas [OK]

Hint: Separate columns with commas in SELECT [OK]

Common Mistakes:

• Omitting commas between columns
• Adding unnecessary WHERE clause
• Misspelling keywords

5. You have a table products with columns product_id, name, and price. You want to find all products priced between 10 and 20 inclusive. Which query correctly does this?

hard

A. SELECT name FROM products WHERE price >= 10 OR price <= 20;

B. SELECT name FROM products WHERE price > 10 OR price < 20;

C. SELECT name FROM products WHERE price BETWEEN 10 AND 20;

D. SELECT name FROM products WHERE price = 10 AND price = 20;

Solution

1. Step 1: Understand the BETWEEN operator

   BETWEEN checks if a value is within a range inclusive of the boundaries, so price BETWEEN 10 AND 20 means price ≥ 10 and ≤ 20.

2. Step 2: Compare other options

   SELECT name FROM products WHERE price >= 10 OR price <= 20; uses OR which selects nearly all products; SELECT name FROM products WHERE price > 10 OR price < 20; uses OR which includes prices outside the range; SELECT name FROM products WHERE price = 10 AND price = 20; checks impossible condition price = 10 AND price = 20 simultaneously.

3. Final Answer:

   SELECT name FROM products WHERE price BETWEEN 10 AND 20; -> Option C

4. Quick Check:

   BETWEEN includes range boundaries [OK]

Hint: Use BETWEEN for inclusive range filtering [OK]

Common Mistakes:

• Using OR instead of AND for range
• Confusing BETWEEN with equality
• Checking impossible conditions with AND