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create table Leads ( leadId int not null, created_date datetime, industry varchar(10), PRIMARY KEY (leadId) ); create table Accounts ( accountId int not null, created_date datetime, revenue_range varchar(10), leadId int not null, FOREIGN KEY (leadId) REFERENCES Leads(leadId) ); insert into Leads values (1, '2020-01-01', 'a'), (2, '2020-01-02', 'b'), (3, '2020-01-03', 'c'), (4, '2020-02-01', 'd'), (5, '2020-03-01', 'e'); insert into Accounts values (1, '2020-01-03', '1k', 1), (2, '2020-03-10', '2k', 5), (3, '2020-02-03', '3k', 2); select -- l.leadId, -- l.created_date as LeadCreatedDate, -- a.created_date as AccountCreatedDate, -- ABS is used because it returns with minus sign AVG(ABS(DATEDIFF(l.created_date, a.created_date))) as AvgDifferenceInDaysBetweenCreation from Leads as l inner join Accounts as a on l.leadId = a.leadId;