Refs #8577 - documentation enhanced for behaviour of SELECT '_test'.

Documentation/Manual.rst

@@ -5503,22 +5503,38 @@ Processing of columns in the SQL result
 ---------------------------------------
 
 * The content of all columns of all rows will be printed sequentially, without separator (except one is defined).
-* Rows with `special-column-names`_  will be rendered first and the output is taken as content.
+* Rows with `special-column-names`_  will be rendered internally by QFQ and the QFQ output of such processing (if there
+  is any) is taken as the content.
 
 .. _special-column-names:
 
 Special column names
 --------------------
 
+QFQ typically don't care about the content of any SQL-Query - it just copy the content to the output (=Browser).
+
+One exception are columns, whose name starts with '_'.  E.g.::
+
+  10.sql = SELECT 'All', 'cats' AS red, 'are' AS _green, 'grey in the night' AS _link
+
+* The first and second column are regular columns. No QFQ processing.
+* The third column (alias name 'green') is no QFQ special column name, but has an '_' at the beginning: this column
+  content will be hidden.
+* The fourth column (alias name 'link') uses a QFQ special column name. Here, only in this example, it has no
+  further meaning.
+
+Summary:
+
 * Special column names always start with '_'.
-* Column names, which start with a '_' and which are not reserved (=special column name), will not be printed. Nevertheless,
-  access to it via the {{<level>.<column>}} variable (without '_') are possible.
-* The input parameters for the processing function are stored as column values.
-* Single parameters are delimited by the '|' character.
-* Parameters are identified by the function either
-
-  * by their **order**
-  * or by a **one character qualifier** followed by the ':' character, placed in front of the actual parameter value.
+* Columns starting with a '_' but not defined as as QFQ special column name are hidden(!) - in other words: they are
+  not **printed** as output.
+* SQL hint: If there is no column alias defined, the column name becomes the value
+  of the first row. E.g. if the first selected row contains a  '_' as the first
+  character, the column name becomes '_....' - which will be hidden! To be safe: always define an alias!
+* Access to 'hidden' columns via {{<level>.<column>}} or {{<column>:R}} are possible and often used.
+* Important: to reference a column, the name has to be given without '_'. E.g. `SELECT 'cat' AS _pet` will be used with
+  `{{pet:R}}` (notice the missing '_').
+* For 'special column names': the column name together with the value controls how QFQ processes the column.
 
 +------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
 | Reserved column name   | Purpose                                                                                                                                                                                     |