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How to Query Empty String In Postgresql?

3 minutes read

To query for empty strings in PostgreSQL, you can use the following SQL statement:

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SELECT * FROM table_name WHERE column_name = '';


This query selects all rows from the specified table where the value in the specified column is an empty string. You can replace table_name with the actual name of your table and column_name with the name of the column you want to check for empty strings.


What is the behavior of empty string comparisons in PostgreSQL?

In PostgreSQL, when comparing empty strings ('') with other strings, the behavior is as follows:

  1. An empty string ('') is considered equal to another empty string ('').
  2. An empty string ('') is considered less than any non-empty string.
  3. A non-empty string is considered greater than an empty string ('').
  4. When comparing two non-empty strings, the comparison is based on the ASCII values of the characters in the strings.


It is important to note that PostgreSQL follows the SQL standard in its handling of empty string comparisons.


How to query for strings with multiple spaces in PostgreSQL?

To query for strings with multiple spaces in PostgreSQL, you can use the ~ operator with a regular expression that matches multiple spaces. Here is an example query:

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SELECT * 
FROM your_table 
WHERE your_column ~ ' {2,}';


This query will return rows where the your_column has two or more consecutive spaces. The regular expression {2,} specifies that we are looking for two or more spaces in a row.


You can adjust the regular expression to match a specific number of spaces or a range of spaces as needed for your query.


How to query for strings with whitespace characters in PostgreSQL?

To query for strings with whitespace characters in PostgreSQL, you can use the LIKE operator along with the % wildcard to match any sequence of characters that contains whitespace. You can use the following query:

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SELECT * FROM table_name
WHERE column_name LIKE '% %';


This query will return all rows where the column_name contains at least one whitespace character. You can also use the ESCAPE clause to search for specific whitespace characters, such as tabs or newlines.

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SELECT * FROM table_name
WHERE column_name LIKE E'%\t%'; -- search for tabs



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SELECT * FROM table_name
WHERE column_name LIKE E'%\n%'; -- search for newlines


You can also use the POSITION function to search for whitespace characters in a string:

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SELECT * FROM table_name
WHERE POSITION(' ' IN column_name) > 0;


This query will return all rows where the column_name contains a whitespace character.


How to trim whitespace characters from strings in PostgreSQL?

In PostgreSQL, the TRIM() function can be used to remove leading and trailing whitespace characters from a string. Here's how you can use it:

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SELECT TRIM('  hello  '); -- Output: 'hello'
SELECT TRIM(BOTH ' ' FROM '  hello  '); -- Output: 'hello'
SELECT TRIM(LEADING ' ' FROM '  hello  '); -- Output: 'hello  '
SELECT TRIM(TRAILING ' ' FROM '  hello  '); -- Output: '  hello'


You can replace ' ' with other characters like tabs ('\t') or newlines ('\n') if you need to remove those specific characters from the string.

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