LOCATE and INSTR in MySQL

Introduction

LOCATE and INSTR help find where a smaller piece of text appears inside a bigger text. This is useful to check if a word or letter is inside a sentence and where it starts.

You want to find the position of a word in a sentence stored in a database.

You need to check if a certain character exists in a text column.

You want to extract or work with parts of text based on where a word appears.

You want to filter rows where a substring appears in a column.

You want to count or analyze text data by locating specific patterns.

Syntax

MySQL

LOCATE(substring, string[, start_position])
INSTR(string, substring)

LOCATE returns the position of the first occurrence of substring in string, starting at start_position if given (default is 1).

INSTR returns the position of the first occurrence of substring in string, but does not support a start position argument.

Examples

Finds where 'cat' starts in 'concatenate'. Result is 4 because 'cat' starts at the 4th letter.

MySQL

SELECT LOCATE('cat', 'concatenate');

Starts searching for 'cat' from the 5th letter. Result is 0 because 'cat' does not appear after position 5.

MySQL

SELECT LOCATE('cat', 'concatenate', 5);

Finds where 'cat' starts in 'concatenate'. Result is 4, same as LOCATE without start position.

MySQL

SELECT INSTR('concatenate', 'cat');

Searches for 'z' in 'hello world'. Result is 0 because 'z' is not found.

MySQL

SELECT INSTR('hello world', 'z');

Sample Program

This query finds the position of 'day' in the sentence using both LOCATE and INSTR functions.

MySQL

SELECT
  LOCATE('day', 'Today is a sunny day') AS locate_result,
  INSTR('Today is a sunny day', 'day') AS instr_result;

Important Notes

Both LOCATE and INSTR return 0 if the substring is not found.

LOCATE allows you to specify where to start searching, INSTR does not.

Positions start at 1, not 0, meaning the first character is position 1.

Summary

LOCATE and INSTR find the position of a substring inside a string.

LOCATE can start searching from a specific position; INSTR always starts at the beginning.

If the substring is not found, both return 0.

Practice

1. What does the MySQL function LOCATE('cat', 'concatenate') return?

easy

A. 0

B. 4

C. 1

D. 7

Solution

1. Step 1: Understand LOCATE function behavior

   LOCATE returns the position of the first occurrence of the substring in the string, starting from the beginning by default.

2. Step 2: Find 'cat' in 'concatenate'

   The substring 'cat' starts at the 4th character in 'concatenate'.

3. Final Answer:

   4 -> Option B

4. Quick Check:

   LOCATE('cat', 'concatenate') = 4 [OK]

Hint: LOCATE returns first position of substring, counting from 1 [OK]

Common Mistakes:

• Counting from 0 instead of 1
• Confusing LOCATE with INSTR starting position
• Assuming substring not found returns NULL

2. Which of the following is the correct syntax to find the position of 'dog' in the string 'bulldog' starting search from position 5 using LOCATE?

easy

A. LOCATE('dog', 'bulldog') FROM 5

B. LOCATE('dog', 5, 'bulldog')

C. LOCATE('bulldog', 'dog', 5)

D. LOCATE('dog', 'bulldog', 5)

Solution

1. Step 1: Recall LOCATE syntax

   The correct syntax is LOCATE(substring, string, start_position).

2. Step 2: Match syntax with options

   LOCATE('dog', 'bulldog', 5) matches the correct order: substring 'dog', string 'bulldog', start position 5.

3. Final Answer:

   LOCATE('dog', 'bulldog', 5) -> Option D

4. Quick Check:

   LOCATE(substring, string, start) correct syntax [OK]

Hint: LOCATE syntax: substring, string, start position [OK]

Common Mistakes:

• Swapping string and substring order
• Using incorrect argument order
• Trying to use FROM keyword

3. What is the output of this query?

SELECT INSTR('hello world', 'o');

medium

A. 6

B. 8

C. 0

D. 5

Solution

1. Step 1: Understand INSTR function behavior

   INSTR returns the position of the first occurrence of the substring in the string, starting from the beginning.

2. Step 2: Find 'o' in 'hello world'

   The first 'o' appears at the 6th character in 'hello world'.

3. Final Answer:

   6 -> Option A

4. Quick Check:

   INSTR('hello world', 'o') = 6 [OK]

Hint: INSTR always starts searching from beginning [OK]

Common Mistakes:

• Choosing position of second 'o' instead of first
• Confusing INSTR with LOCATE with start position
• Assuming zero-based index

4. Identify the error in this query:

SELECT LOCATE('test', 'testing', 0);

medium

A. Start position cannot be zero; must be >= 1

B. String and substring arguments are swapped

C. No error; query is valid

D. LOCATE does not accept three arguments

Solution

1. Step 1: Check LOCATE start position rules

   The start position argument can be less than 1; it is treated as 1 with no error.

2. Step 2: Analyze the query

   The query uses 0 as start position, which is valid; the search starts from position 1.

3. Final Answer:

   No error; query is valid -> Option C

4. Quick Check:

   LOCATE start position <1 clamped to 1 [OK]

Hint: LOCATE start_position <1 is allowed, search starts from 1 [OK]

Common Mistakes:

• Believing start position must be >=1 (it's clamped if less)
• Confusing argument order
• Assuming LOCATE uses zero-based indexing

5. Given the string 'abracadabra', which query returns the position of the second occurrence of 'a' using LOCATE?

hard

A. SELECT LOCATE('a', 'abracadabra', LOCATE('a', 'abracadabra') + 1);

B. SELECT INSTR('abracadabra', 'a', 2);

C. SELECT LOCATE('a', 'abracadabra', 5);

D. SELECT LOCATE('a', 'abracadabra', 1);

Solution

1. Step 1: Find first occurrence of 'a'

   LOCATE('a', 'abracadabra') returns 1, the position of the first 'a'.

2. Step 2: Use LOCATE with start position after first 'a'

   To find the second 'a', start searching from position 2 (1 + 1) using LOCATE('a', 'abracadabra', 2).

3. Step 3: Combine steps in one query

   Use nested LOCATE: LOCATE('a', 'abracadabra', LOCATE('a', 'abracadabra') + 1) to get second occurrence.

4. Final Answer:

   SELECT LOCATE('a', 'abracadabra', LOCATE('a', 'abracadabra') + 1); -> Option A

5. Quick Check:

   Nested LOCATE finds second occurrence [OK]

Hint: Use nested LOCATE with start = first position + 1 [OK]

Common Mistakes:

• Using INSTR with start position (not supported)
• Starting search at wrong position
• Not adding 1 to first occurrence position