Why we can’t make Salesforce Query in the Constructor?

Hi Suraj

No you can’t perform any DML in Constructor.

If you want to Perform DML then you have to write a function in class and call this functon by action attribute of <apex:page action=”YOUR Funciton Name”>

for more infohttp://salesforceworld4u.blogspot.in/2014/08/how-to-do-dml-operation-when-page-loads.html

Hope it may help you