Working with parameters Delphi XE7 Firedac - mysql

Question

Any assistance here would be great.
I am trying to use parameters to dynamically change 'ORDER BY'
Below is the code I have tried but despite following the documentation I still get an error '[FIREDAC][PHYS][MYSQL] You have an error in your SQL syntax ... near "ORDER BY some_field" at line 4'
I have set ParamCreate to True
My database is MySQL
FDQuery1.Close;
FDQuery1.SQL.Clear;
FDQuery1.SQL.Add('SELECT *');
FDQuery1.SQL.Add('FROM my_table');
FDQuery1.SQL.Add('LIMIT 1000');
FDQuery1.SQL.Add(':id');
FDQuery1.ParamByName('id').AsString := 'ORDER BY some_field';
FDQuery1.Open;

Answer 1

You did not cite the exception message as it shows up. Here is the original message
[FireDAC][Phys][MySQL] You have an error in your SQL syntax ... near ''ORDER BY some_field'' at line 4.
compare to your cite
[FIREDAC][PHYS][MYSQL] You have an error in your SQL syntax ... near "ORDER BY some_field" at line 4
To avoid this for the future just press CTRL+C on the focused exception window and the complete message is inside your clipboard and can be pasted wherever you like
Now reading this, the error is now very clear.
You expect to get a statement like this
SELECT *
FROM my_table
LIMIT 1000
ORDER BY some_field
But using the parameter you will get the following statement
SELECT *
FROM my_table
LIMIT 1000
'ORDER BY some_field'
and that is exactly what the exception message is telling you.
Just check the exception message with the previous statement
... near 'ORDER BY some_field' at line 4.
and
... near ''ORDER BY some_field'' at line 4.
As a conclusion it is not possible to change the statement itself using parameters. You can only pass values as parameters for the statement.
And the correct statement should be anyway
SELECT *
FROM my_table
ORDER BY some_field
LIMIT 1000

Answer 2

Don't know if this helps.
But you can use the 'Macros' property of TFDQuery, like Parameter that are identified by the ':', the Macros are identified bye the '!', You can also combine Macros and Params. The Macros property works almost as the Params property. Use the TFDQuery.MacroByname to assign a Macro Value, and use the TFDQuery.MacroByname('MacroName').AsRaw to assign a string As-Is.
So your query should look like:
FDQuery1.Close;
FDQuery1.SQL.Text := 'SELECT * FROM !TABLE_NAME !WHERE_CLAUSE !ORDERBY_CLAUSE';
FDQuery.MacroByname('Table_name').AsRaw := 'my_table';
FDQuery.MacroByname('Where_clause').AsRaw := 'WHERE field1 = :ID_Value';
FDQuery.MacroByname('OrderBy_clause').AsRaw := 'ORDER BY field1';
FDQuery.ParamByname('ID_Value').AsInteger := 1;
FDQuery1.Open;
Hope this helps

Answer 3

you need very simple SQL query:
FDQuery1.Close;
FDQuery1.SQL.Text := 'SELECT * FROM my_table';
FDQuery1.Open;
To set a limit of the record count, you can use property of the FDQuery1:
FDQuery1.FetchOptions.RecsMax := 1000;
To sort values you can use
FDQuery1.IndexFieldNames = 'field_name'
or
FDQuery1.IndexFieldNames = 'field_one_name;field_two_name'
instead of your code.

Answer 4

Edit: Oh, I didn't actually answer your question. So for FireDac, you can just set the FDQuery1.IndexFieldNames property to the name of the field/s you want to order by. No need to close and re-open your query, or change the SQL.
Previous answer: You cannot pass SQL code [EDIT: including ORDER BY etc.] in parameters, only parameter values, i.e. integers, strings etc.
This principle is extremely important, otherwise you could pass e.g.
FDQuery1.ParamByName('id').AsString := '; TRUNCATE TABLE my_table';
Executing your query would then delete everything in the table instead of doing what it's supposed to do. Or with a bit more work, your same query could return passwords, credit card numbers or whatever else is in your database. This would have been a huge vulnerability, and is known as SQL Injection. Please see for example:
http://www.w3schools.com/sql/sql_injection.asp
http://sqlmap.org/
http://hackaday.com/2014/09/01/gaining-access-to-the-oculus-developer-database

Answer 5

RXLIB has this functionality. It has the MACRO opttions, where you can write a code like this:
Select %fields_
from %table_
where %condition_
order by %order_
BUT it´s only for use qith BDE.
Will be wonderfull if someone rewrite the code to work with ADO ou FIREDAC.