Condition on two columns to count visits

[sharasridhar]

I have two tables:

1. Readmission

The readmission table contains readmission visits with columns Readmit_PatientID, Readmit_VisitID, Readmit_AdmitDateTime, PrevAdmit_VisitID, PrevAdmit_DischargeDateTime.

Readmit_VisitID is unique in the Readmission table.

2. All_Visits

The All Visits table contains all the visits with columns Patient_ID, VisitServiceDateTime, VisitID. 

VisitID is unique in the All_Visits table.

The two tables are joined by Readmission.Prev_VisitID and AllVisits.VisitID

I need to find the number of Visits between the PrevAdmit_DischargeDateTime and Readmit_AdmitDateTime.