FAQ about whitespace detection.

docs/categories-of-data-quality-checks/how-to-detect-blank-and-whitespace-values.md

@@ -2,7 +2,7 @@
 title: How to detect whitespace and null value placeholders
 ---
 # How to detect whitespace and null value placeholders
-Read this guide to learn how to detect data quality issues in text columns containing spaces, tabs, or special texts equivalent to a null value.
+Read this guide to learn how to detect whitespaces, such as spaces, tabs, or special texts equivalent to a null value in text columns using SQL checks.
 
 The data quality checks for detecting whitespace and empty value placeholders are configured in the `whitespace` category in DQOps.
 
@@ -233,6 +233,107 @@ The reference section provides YAML code samples that are ready to copy-paste to
 the parameters reference, and samples of data source specific SQL queries generated by [data quality sensors](../dqo-concepts/definition-of-data-quality-sensors.md)
 that are used by those checks.
 
+## FAQ
+The questions and answers for popular questions related to detecting whitespace characters.
+
+### What is a whitespace character
+A whitespace character is any character that represents a blank space, such as a regular space, a tab, or a new line. 
+These characters are often invisible to the human eye but can affect how data is processed and interpreted by databases.
+When users are previewing column values, the columns that contain values ending with whitespace look the same as trimmed values. 
+However, the values don't match, and a query that uses a filter in the WHERE clause in SQL will not find all rows.
+
+### What is the valid name, "whitespace" or "white space"
+Both "whitespace" and "white space" are used, but "whitespace" is generally considered the more technically correct and contemporary term, especially in computing and data management contexts.
+You'll find it used more frequently in technical documentation and programming.
+
+### How to check if a column value has space in SQL
+To check for spaces in a SQL column, use the LIKE operator (e.g., column_name `LIKE '% %'`) or remove spaces with TRIM and compare the result to the original column
+(e.g., `WHERE TRIM(column_name) <> column_name`). 
+Remember that spaces are just one type of whitespace. For a thorough check, consider your database's specific functions or regular expressions.
+
+### How to check if a column value has space in SQL Server
+In SQL Server, you can use all generic methods of detecting whitespace characters mentioned in the previous answer.
+SQL Server also offers specialized functions like `CHARINDEX` to locate the position of a space within a string, giving you more precise control.
+
+The following query will find a space character using TransactSQL in SQL Server:
+
+```sql
+SELECT column_name
+FROM your_table
+WHERE CHARINDEX(' ', column_name) > 0
+```
+
+### What is the difference between using IS NULL or finding a whitespace in SQL?
+`IS NULL` checks if a column value is explicitly defined as *NULL*, meaning it has no value at all. This is a specific state recognized by the database.
+On the other hand, finding a whitespace (using `LIKE`, `TRIM`, etc.) means looking for characters like spaces, tabs, or newlines that represent "blank" space.
+These are actual characters stored in the database, even though they might appear invisible.
+
+Here's why this difference matters:
+
+*  **Database Storage**: Some databases, notably **Oracle**, have a unique behavior. They might not store truly empty string values. 
+   Instead, they represent them as *NULL* values. This can lead to unexpected results if you're searching for empty strings but the database treats them as NULLs.
+
+*  **Configuration**:  Both Oracle and SQL Server allow you to configure how the database handles comparisons between empty strings and *NULL* values. 
+   This setting can affect query results and indexing. For instance, in Oracle, you can use the `ANSI_NULLS` setting to control this behavior.
+   In SQL Server, this is influenced by the `SET ANSI_NULLS` option. 
+
+*  **Sorting and Indexing**: Empty strings and *NULL* values are treated differently in sorting operations and index construction. 
+   This can impact the performance and organization of your data.
+
+### How PostgreSQL handles NULL or empty value in GROUP BY?
+PostgreSQL has a clear and consistent way of handling *NULL* and empty values in `GROUP BY`:
+
+* *NULL* values are grouped together: If a column has multiple *NULL* values, they will be treated as a single group in the `GROUP BY` result.
+
+* Empty strings are grouped together: Similar to *NULL*s, all empty strings (`''`) are considered the same and grouped into a single group.
+
+* *NULL* and empty strings are distinct: Importantly, PostgreSQL distinguishes between *NULL* and empty strings. They are treated as separate groups in the `GROUP BY` clause.
+
+### How to represent a tab character in SQL?
+To represent a tab character in SQL, use `CHAR(9)` function. The ASCII code for a tab character is 9.
+Some databases may allow you to directly insert a tab character using the Tab key or an escape sequence like `\t`, but CHAR(9) is the most reliable method in databases
+that support this function.
+
+Here are the examples of using the `CHAR` or similar functions in the most popular databases:
+
+* **SQL Server**: `CHAR()` is a standard function.
+* **Oracle**: Oracle uses `CHR()` to achieve the same result.
+* **PostgreSQL**: `CHAR()` is fully supported.
+* **MySQL**: `CHAR()` is a standard function.
+* **IBM DB2**: `CHAR()` is part of the standard SQL functions.
+* **SQLite**: `CHAR()` is available.
+
+### How to remove whitespaces around a text in SQL?
+To remove whitespaces around text in SQL, you can use the `TRIM()` function. It removes leading and trailing whitespace characters (spaces, tabs, newlines, etc.) from a string.
+
+Here's a simple example:
+
+```sql
+SELECT TRIM('  This has extra spaces.  ')
+```
+
+This will return:
+
+```text
+This has extra spaces.
+```
+
+### How to remove all spaces inside a text using SQL?
+While `TRIM()` function removes spaces around text, to remove all spaces within the text, you'll need a different approach.  Most databases offer a function like `REPLACE()`.
+This function replaces all occurrences of a specified character with another character. In this case, you'd replace all spaces (`' '`) with an empty string (`''`).
+
+Here's how it works:
+
+```sql
+SELECT REPLACE(' This has extra spaces. ', ' ', '')
+```
+
+This will return:
+
+```text
+Thishasextraspaces.
+```
+
 ## What's next
 - Learn how to [run data quality checks](../dqo-concepts/running-data-quality-checks.md#targeting-a-category-of-checks) filtering by a check category name
 - Learn how to [configure data quality checks](../dqo-concepts/configuring-data-quality-checks-and-rules.md) and apply alerting rules