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

Concept Flow - SELECT specific columns

Start Query

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Parse SELECT clause

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Identify columns to retrieve

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Scan table rows

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Extract specified columns from each row

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Return result set with selected columns

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End Query

The query starts by reading which columns to select, then scans the table rows, extracts only those columns, and returns them as the result.

Execution Sample

SQL

SELECT name, age FROM users;

This query retrieves only the 'name' and 'age' columns from every row in the 'users' table.

Execution Table

Step	Action	Current Row Data	Columns Extracted	Result Set So Far
1	Start query execution	N/A	N/A	[]
2	Read first row	{id:1, name:'Alice', age:30, city:'NY'}	name, age	[{name:'Alice', age:30}]
3	Read second row	{id:2, name:'Bob', age:25, city:'LA'}	name, age	[{name:'Alice', age:30}, {name:'Bob', age:25}]
4	Read third row	{id:3, name:'Carol', age:27, city:'Chicago'}	name, age	[{name:'Alice', age:30}, {name:'Bob', age:25}, {name:'Carol', age:27}]
5	No more rows	N/A	N/A	[{name:'Alice', age:30}, {name:'Bob', age:25}, {name:'Carol', age:27}]

💡 All rows processed, query returns result set with only 'name' and 'age' columns.

Variable Tracker

Variable	Start	After Row 1	After Row 2	After Row 3	Final
result_set	[]	[{name:'Alice', age:30}]	[{name:'Alice', age:30}, {name:'Bob', age:25}]	[{name:'Alice', age:30}, {name:'Bob', age:25}, {name:'Carol', age:27}]	[{name:'Alice', age:30}, {name:'Bob', age:25}, {name:'Carol', age:27}]

Key Moments - 2 Insights

Why does the result only include 'name' and 'age' and not other columns like 'id' or 'city'?

Because the SELECT clause specifies only 'name' and 'age' columns, the query extracts only these columns from each row as shown in execution_table rows 2-4.

What happens if the table has more rows than shown?

The query continues scanning each row, extracting only the specified columns, and appends them to the result set until no rows remain, as indicated in execution_table row 5.

Visual Quiz - 3 Questions

Look at the execution table, what columns are extracted from the second row?

A id, name

B name, age

C age, city

D id, city

💡 Hint

Check the 'Columns Extracted' column in execution_table row 3.

At which step does the query finish reading all rows?

A Step 5

B Step 4

C Step 3

D Step 2

💡 Hint

Look for the step where 'No more rows' is indicated in the 'Action' column.

If we add 'city' to the SELECT clause, how would the 'Columns Extracted' change at step 2?

A id, city

B name, age

C name, age, city

D id, name, age

💡 Hint

Adding 'city' means it will be included in the extracted columns for each row.

Concept Snapshot

SELECT specific columns syntax:
SELECT column1, column2 FROM table_name;

- Retrieves only listed columns from all rows.
- Result set contains fewer columns than the full table.
- Useful to get just needed data, improving clarity and performance.

Full Transcript

This visual execution trace shows how a SQL query with SELECT specific columns works. The query starts by reading which columns to select, then scans each row in the table. For each row, it extracts only the specified columns and adds them to the result set. This continues until all rows are processed. The final output contains only the chosen columns for every row. This helps to get just the data you want from a table without extra information.