Difference between WQL and SQL in SCCM

Below are few points which make difference between WQL and SQL :

1. WMI provides its own query language that allows you to query managed objects as data providers
2. WMI Query Language (WQL) is essentially a subset of SQL (Structured Query Language) with minor semantic changes.
3. Unlike SQL, WQL does not provide statements for inserting, deleting, or updating data and does not support stored procedures.
4. WQL does have extensions that support WMI events and other features specific to WMI. WQL is the basis for Configuration Manager queries, whereas SQL is used for ConfigMgr reports.
5. One important advantage of WQL is that a WQL query can return WMI objects as well as specific properties.
6. Because management applications such as the Configuration Manager console interact with WMI objects, WQL queries can return result sets that you can use within the ConfigMgr infrastructure.

Eg: Configuration Manager collections are based on WQL queries.

Ref: http://systemcentersupport.blogspot.com/2010/01/what-is-wmi-query-langauge-or-wql.html