SELECT all columns in SQL - Step-by-Step Execution

Concept Flow - SELECT all columns

Start Query

↓

Parse SELECT *

↓

Identify Table

↓

Retrieve All Columns

↓

Return All Rows with All Columns

↓

End Query

The query starts by reading SELECT * which means all columns from the specified table are retrieved and returned.

Execution Sample

SQL

SELECT * FROM employees;

This query retrieves every column and every row from the employees table.

Execution Table

Step	Action	Evaluation	Result
1	Parse query	SELECT * FROM employees	Ready to execute
2	Identify table	employees	Table found with columns: id, name, role, salary
3	Retrieve columns	* means all columns	Columns selected: id, name, role, salary
4	Fetch rows	All rows from employees	Rows fetched: 3
5	Return result	All columns and rows	Result set with 3 rows and 4 columns
6	End query	Query complete	Execution finished

💡 All rows and all columns retrieved, query execution ends.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
table	undefined	employees	employees	employees	employees
columns	undefined	undefined	id, name, role, salary	id, name, role, salary	id, name, role, salary
rows	undefined	undefined	undefined	3 rows fetched	3 rows fetched
result_set	undefined	undefined	undefined	undefined	3 rows x 4 columns

Key Moments - 2 Insights

Why does SELECT * return all columns instead of just one?

SELECT * is a shortcut that tells the database to include every column from the table, as shown in execution_table step 3 where all columns are selected.

Does SELECT * limit the number of rows returned?

No, SELECT * returns all rows by default unless a WHERE clause is added. Execution_table step 4 shows all rows are fetched.

Visual Quiz - 3 Questions

Look at the execution table, what columns are selected at step 3?

A No columns selected yet

B id, name, role, salary

C Only id and name

D Only salary

💡 Hint

Check the 'Retrieve columns' action in execution_table step 3.

At which step does the query fetch all rows from the table?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look for 'Fetch rows' action in execution_table.

If the table had 5 columns instead of 4, how would the result set change?

A It would return 5 columns

B It would still return 4 columns

C It would return only 1 column

D It would return no columns

💡 Hint

SELECT * means all columns, so adding columns increases the count in result_set.

Concept Snapshot

SELECT * syntax:
SELECT * FROM table_name;

- '*' means all columns
- Returns all rows by default
- Useful to quickly get full table data
- Can be combined with WHERE to filter rows

Full Transcript

This visual execution shows how the SQL query SELECT * FROM employees works step-by-step. First, the query is parsed and the employees table is identified. Then, the * symbol tells the database to select all columns from the table. Next, all rows are fetched from employees. Finally, the full result set with all columns and rows is returned. This helps beginners see how SELECT * retrieves complete data from a table.

Practice

1. What does the SQL query SELECT * FROM employees; do?

easy

A. It deletes all data from the employees table.

B. It creates a new employees table.

C. It updates all columns in the employees table.

D. It retrieves all columns and all rows from the employees table.

Solution

1. Step 1: Understand the SELECT * syntax

   The asterisk (*) means select all columns from the specified table.

2. Step 2: Apply to the employees table

   The query fetches every column and every row from the employees table.

3. Final Answer:

   It retrieves all columns and all rows from the employees table. -> Option D

4. Quick Check:

   SELECT * = all columns and rows [OK]

Hint: SELECT * means get everything from the table [OK]

Common Mistakes:

• Thinking SELECT * deletes or updates data
• Confusing SELECT * with creating tables
• Assuming it selects only one column

2. Which of the following is the correct syntax to select all columns from a table named products?

easy

A. SELECT * FROM products;

B. SELECT products.*;

C. SELECT columns FROM products;

D. SELECT ALL FROM products;

Solution

1. Step 1: Recall the correct SELECT syntax

   The correct way to select all columns is using SELECT * FROM table_name.

2. Step 2: Check each option

   SELECT * FROM products; matches the correct syntax exactly.

3. Final Answer:

   SELECT * FROM products; -> Option A

4. Quick Check:

   SELECT * FROM table = correct syntax [OK]

Hint: Use SELECT * FROM table_name to get all columns [OK]

Common Mistakes:

• Using SELECT ALL instead of SELECT *
• Forgetting FROM keyword
• Trying to select 'columns' literally

3. Given the table students with columns id, name, and age, what will the query SELECT * FROM students WHERE age > 20; return?

medium

A. All students regardless of age.

B. Only the age column for students older than 20.

C. All columns for students whose age is greater than 20.

D. An error because * cannot be used with WHERE.

Solution

1. Step 1: Understand SELECT * with WHERE clause

   SELECT * fetches all columns, and WHERE filters rows based on condition.

2. Step 2: Apply condition age > 20

   Only rows where age is greater than 20 are returned, with all columns shown.

3. Final Answer:

   All columns for students whose age is greater than 20. -> Option C

4. Quick Check:

   SELECT * + WHERE filters rows [OK]

Hint: SELECT * returns all columns; WHERE filters rows [OK]

Common Mistakes:

• Thinking WHERE limits columns instead of rows
• Assuming * selects only one column
• Believing * cannot be used with WHERE

4. Identify the error in this SQL query: SELECT * employees;

medium

A. Table name should be in quotes.

B. Missing FROM keyword before table name.

C. Using * is not allowed in SELECT.

D. SELECT cannot be used without WHERE.

Solution

1. Step 1: Check SQL SELECT syntax

   The correct syntax requires FROM before the table name.

2. Step 2: Identify missing keyword

   The query misses FROM between SELECT * and employees.

3. Final Answer:

   Missing FROM keyword before table name. -> Option B

4. Quick Check:

   SELECT * FROM table is correct syntax [OK]

Hint: Always use FROM before table name in SELECT [OK]

Common Mistakes:

• Omitting FROM keyword
• Thinking * is invalid
• Adding unnecessary quotes around table name

5. You want to quickly check all data in a large table named orders. Which query is best to avoid slowing down your database?

hard

A. SELECT * FROM orders LIMIT 10;

B. SELECT * FROM orders;

C. SELECT order_id FROM orders;

D. DELETE FROM orders WHERE order_id < 10;

Solution

1. Step 1: Understand SELECT * impact on large tables

   SELECT * fetches all rows and columns, which can be slow on big tables.

2. Step 2: Use LIMIT to reduce rows fetched

   Adding LIMIT 10 fetches only 10 rows, making the query faster and safer.

3. Final Answer:

   SELECT * FROM orders LIMIT 10; -> Option A

4. Quick Check:

   LIMIT reduces rows, speeding up SELECT * [OK]

Hint: Use LIMIT with SELECT * on big tables to speed queries [OK]

Common Mistakes:

• Running SELECT * without LIMIT on large tables
• Confusing DELETE with SELECT
• Selecting only one column when all data is needed