String length and position functions in PostgreSQL

Introduction

These functions help you find how long a text is and where a smaller text appears inside a bigger text.

When you want to count how many characters are in a word or sentence.

When you need to find the place of a specific word inside a sentence.

When checking if a certain part of text exists in a bigger text.

When splitting text based on where a word or character appears.

When cleaning or validating text data by length or position.

Syntax

PostgreSQL

LENGTH(string)
POSITION(substring IN string)

LENGTH returns the number of characters in the string.

POSITION returns the starting position of the substring inside the string, or 0 if not found.

Examples

Finds the number of characters in 'Hello World'.

PostgreSQL

SELECT LENGTH('Hello World');

Finds where the word 'World' starts in the text.

PostgreSQL

SELECT POSITION('World' IN 'Hello World');

Returns 0 because the string is empty.

PostgreSQL

SELECT LENGTH('');

Returns 0 because 'x' is not found in 'abc'.

PostgreSQL

SELECT POSITION('x' IN 'abc');

Sample Program

This query finds the length of the word 'Database' and the position where 'base' starts inside it.

PostgreSQL

SELECT LENGTH('Database') AS length_of_word,
       POSITION('base' IN 'Database') AS position_of_substring;

Important Notes

Positions start counting at 1, not 0.

If the substring is not found, POSITION returns 0.

LENGTH counts all characters including spaces and punctuation.

Summary

Use LENGTH to count characters in a string.

Use POSITION to find where a substring starts.

These functions help you understand and work with text data.

Practice

1. What does the LENGTH function do in PostgreSQL?

easy

A. Splits a string into an array

B. Finds the position of a substring inside a string

C. Converts a string to uppercase

D. Returns the number of characters in a string

Solution

1. Step 1: Understand LENGTH function purpose

   The LENGTH function counts how many characters are in a given string.

2. Step 2: Compare with other options

   POSITION finds substring location, UPPER changes case, SPLIT divides string. Only LENGTH counts characters.

3. Final Answer:

   Returns the number of characters in a string -> Option D

4. Quick Check:

   LENGTH = character count [OK]

Hint: LENGTH counts characters, POSITION finds substring start [OK]

Common Mistakes:

• Confusing LENGTH with POSITION
• Thinking LENGTH changes string case
• Assuming LENGTH splits strings

2. Which of the following is the correct syntax to find the position of substring 'cat' in string 'concatenate' using PostgreSQL?

easy

A. LENGTH('cat' IN 'concatenate')

B. POSITION('concatenate' IN 'cat')

C. POSITION('cat' IN 'concatenate')

D. FIND('cat', 'concatenate')

Solution

1. Step 1: Recall POSITION syntax

   POSITION(substring IN string) finds where substring starts in string.

2. Step 2: Check each option

   POSITION('cat' IN 'concatenate') matches syntax. POSITION('concatenate' IN 'cat') reverses arguments. LENGTH('cat' IN 'concatenate') uses LENGTH wrongly. FIND('cat', 'concatenate') uses non-existent FIND function.

3. Final Answer:

   POSITION('cat' IN 'concatenate') -> Option C

4. Quick Check:

   POSITION(substring IN string) correct syntax [OK]

Hint: Use POSITION('sub' IN 'string') to find substring start [OK]

Common Mistakes:

• Swapping substring and string arguments
• Using LENGTH instead of POSITION
• Using non-PostgreSQL functions like FIND

3. What is the result of this query?

SELECT POSITION('day' IN 'holiday') AS pos, LENGTH('holiday') AS len;

medium

A. pos = 4, len = 7

B. pos = 5, len = 7

C. pos = 0, len = 7

D. pos = 4, len = 6

Solution

1. Step 1: Find POSITION of 'day' in 'holiday'

   'holiday' positions: 1:'h', 2:'o', 3:'l', 4:'i', 5:'d', 6:'a', 7:'y'. 'day' matches positions 5('d')-6('a')-7('y'), so starts at 5 (1-based).

2. Step 2: Calculate LENGTH of 'holiday'

   'holiday' has 7 characters.

3. Final Answer:

   pos = 5, len = 7 -> Option B

4. Quick Check:

   POSITION('day' IN 'holiday') = 5, LENGTH('holiday') = 7 [OK]

Hint: Count characters carefully; POSITION is 1-based [OK]

Common Mistakes:

• Counting POSITION starting at 0
• Miscounting substring start
• Confusing LENGTH with POSITION

4. Identify the error in this query:

SELECT POSITION('test', 'contest') AS pos;

medium

A. Uses comma instead of IN keyword in POSITION function

B. Missing AS keyword for alias

C. POSITION does not accept string literals

D. Syntax is correct, no error

Solution

1. Step 1: Check POSITION function syntax

   POSITION requires syntax POSITION(substring IN string), not POSITION(substring, string).

2. Step 2: Identify error in query

   The query uses comma instead of IN keyword, causing syntax error.

3. Final Answer:

   Uses comma instead of IN keyword in POSITION function -> Option A

4. Quick Check:

   POSITION(substring IN string) correct syntax [OK]

Hint: Use IN keyword, not comma, in POSITION function [OK]

Common Mistakes:

• Using comma instead of IN in POSITION
• Assuming alias AS is mandatory (it's optional)
• Thinking POSITION can't use string literals

5. You want to find the position of the first space character in the string 'Find the position'. Which query will correctly return this position?

hard

A. SELECT POSITION(' ' IN 'Find the position');

B. SELECT LENGTH(' ' IN 'Find the position');

C. SELECT POSITION('space' IN 'Find the position');

D. SELECT POSITION(' ' FROM 'Find the position');

Solution

1. Step 1: Identify correct function usage for substring position

   POSITION(substring IN string) finds where substring starts. We want the space character ' '.

2. Step 2: Evaluate each option

   SELECT POSITION(' ' IN 'Find the position'); uses correct syntax and substring. SELECT LENGTH(' ' IN 'Find the position'); uses LENGTH wrongly. SELECT POSITION('space' IN 'Find the position'); searches for word 'space' which is not present. SELECT POSITION(' ' FROM 'Find the position'); uses FROM instead of IN, which is invalid syntax.

3. Final Answer:

   SELECT POSITION(' ' IN 'Find the position'); -> Option A

4. Quick Check:

   POSITION(' ' IN string) finds first space [OK]

Hint: Use POSITION(' ' IN string) to find first space [OK]

Common Mistakes:

• Using LENGTH instead of POSITION
• Searching for word 'space' instead of character
• Using FROM instead of IN in POSITION