Concatenation with || operator in PostgreSQL - Step-by-Step Execution

Concept Flow - Concatenation with || operator

Start with two strings

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Apply || operator

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Combine strings end-to-end

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Return concatenated string

The || operator takes two strings and joins them together into one longer string.

Execution Sample

PostgreSQL

SELECT 'Hello' || ' ' || 'World' AS greeting;

This query joins three strings: 'Hello', a space, and 'World' into one string 'Hello World'.

Execution Table

Step	Expression Evaluated	Intermediate Result	Action
1	'Hello' || ' '	'Hello '	Concatenate 'Hello' and space
2	'Hello ' || 'World'	'Hello World'	Concatenate previous result with 'World'
3	Return final result	'Hello World'	Output the concatenated string

💡 All parts concatenated, final string returned

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
result	NULL	'Hello '	'Hello World'	'Hello World'

Key Moments - 2 Insights

Why does the space ' ' need to be included explicitly?

Because || just joins strings exactly as they are; it does not add spaces automatically. See execution_table step 1 where 'Hello' and ' ' are joined.

What happens if one side of || is NULL?

The result becomes NULL because concatenating with NULL yields NULL in PostgreSQL. This is not shown here but important to remember.

Visual Quiz - 3 Questions

Look at the execution table, what is the intermediate result after step 1?

A 'HelloWorld'

B 'Hello '

C 'Hello'

D ' World'

💡 Hint

Check the 'Intermediate Result' column in row for step 1

At which step is the final concatenated string formed?

A Step 2

B Step 1

C Step 3

D No final string is formed

💡 Hint

Look at the 'Intermediate Result' after step 2 in the execution table

If we remove the space ' ' from the query, what would the final output be?

A 'Hello '

B 'Hello World'

C 'HelloWorld'

D NULL

💡 Hint

Think about what happens when you concatenate 'Hello' directly with 'World' without space

Concept Snapshot

Concatenation with || operator in PostgreSQL:
- Syntax: string1 || string2
- Joins two strings end-to-end
- Does not add spaces automatically
- NULL on either side results in NULL
- Useful for combining text columns or literals

Full Transcript

This visual execution shows how the PostgreSQL concatenation operator || works. Starting with two strings, the operator joins them end-to-end. In the example, 'Hello' is concatenated with a space ' ', resulting in 'Hello '. Then this result is concatenated with 'World' to form 'Hello World'. The execution table traces each step, showing intermediate results. The variable tracker follows the 'result' variable as it changes. Key moments clarify why spaces must be added explicitly and what happens with NULL values. The quiz tests understanding of intermediate results and final output. This helps beginners see exactly how string concatenation happens step-by-step in SQL.