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Overview

The SUBSTR() function extracts a specific number of characters from a string.

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Syntax

The syntax of the function is illustrated below:

2 Arguments

substr( string, start_position)

3 Arguments

substr( string, start_position, length )
Both syntaxes will have input and return of type string.

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Start Position

The start_position is used as the starting position, specifying the part from where the substring is to be returned. It is written as an integer value.

InputReturn
start_position < 0 ``start_position < stringThe start_position is a given character in the string. The count starts from the first character.
start_position > stringReturns an empty substring.
start_position = negative valueThe count starts from the provided negative value, with subsequent characters yielded as it approaches 0.

If the index is less than or equal to 0, no characters are returned.

Once it exceeds 0, characters from the string are yielded, starting from the first one.

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Length

The length is used to determine the number of characters to be extracted*. *It can be one or more characters.

InputReturn
length = 0Returns an empty substring.
length is not setThe function will start from the specified start_position and end at the last character of the string.
length = negative valueReturns an error.

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Examples

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Case 1: SUBSTR() function with specified start_position & length

In this example, we will set the start_position with the first six characters and have five characters extracted:

SELECT substr('Watermelon',6,5) AS "Fruit";

The updated table is shown below:

Fruit 
-------
 melon

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Case 2: SUBSTR() function with length = 0

The following query will extract a string with length = 0:

SELECT substr('Watermelon',6,0) AS "Fruit";

It will display an empty output as there is no length specified:

Fruit 
-------

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Case 3: SUBSTR() function with length = negative value

Here we will check if the length is specified with a negative value:

SELECT substr('Watermelon',6,-2) AS "Fruit";

Instead of extracting the string from the last characters, it will return an error as seen below:

ERROR:  Length of substring cannot be negative

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Case 4: SUBSTR() function with start_position > string

We know that Watermelon only has ten characters, but this time, we will figure out if the specified start_position is larger than the string’s characters:

SELECT substr('Watermelon',20,2) AS "Fruit";

It will display an empty output as shown below:

Fruit 
-------

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Case 5: SUBSTR() Function with 2 Arguments

In this example, we will set the start_position with the first six characters and have five characters extracted.

SELECT substr('database', 6) AS "Result";

It will display the substring from position 6 output as shown below:

Result 
--------
 ase
STRPOSSUBSTRING