Query of Queries
Query of queries (QoQ) is a technique for re-querying an existing query without another trip to the database.
<!--- query a database in the normal manner ---> <cfquery name="sourceQry" datasource="mydsn"> SELECT * FROM my_db_table </cfquery> <!--- query the above query *object*. (this doesn't make a call to the database.) ---> <cfquery name="newQry" dbtype="query"><!--- the dbtype="query" attribute/value enables QoQ --> SELECT * FROM sourceQry <!--- instead of a real database table name, use the variable name of the source query object ---> </cfquery>
The above example isn't very useful, because
newQry is a straight copy of the source query, but it demonstrates the two requirements of
- The dbtype="query" attribute
- A source query object name (e.g., sourceQry) instead of table name in the FROM clause.
Let's say you have the following database query,
<cfquery name="myQuery" datasource="mydsn"> SELECT Name, Age, Location FROM People </cfquery>
You would now have a list of names, ages and locations for all the people in a query called
Say you want to filter out people under 18 and over 90, but you don't want to hit the database again:
<cfquery name="filteredQuery" dbtype="query"> SELECT Name, Age, Location FROM myQuery WHERE Age >= 18 AND Age <= 90 </cfquery>
filteredQry contains the desired records.
Lucee uses its own SQL implementation for QoQ; when that fails, HSQLDB is tried.
Lucee's SQL implementation is a basic subset of ANSI92, but it is relatively fast. HSQLDB is a more complete SQL implementation, but it is slow, as compared to Lucee's implementation.
Even though under the hood, Lucee handles the fallback to HSQLDB automatically, it still can be useful to know what's possible with the fast Lucee SQL implementation versus the slower, fallback HSQLDB SQL implementation.
Lucee's SQL Implementation
Keywords and Operators
At the time of this writing (Lucee 4.2), the following were supported by Lucee's SQL implementation:
- BETWEEN x AND y
- GROUP BY
- IN ()
- IS NOT NULL
- IS NULL
- NOT IN ()
- NOT LIKE
- ORDER BY
TODO: Flesh this out.
HSQLDB SQL Implementation
This is the fallback for when Lucee's SQL implementation can't handle the QoQ syntax. See the HSQLDB documentation for details.
Lucee Google Groups Post: SQL syntax supported by query-of-queries?