How to get count of consecutive dates

Question

For example there is some table with dates:

2015-01-01
2015-01-02
2015-01-03
2015-01-06
2015-01-07
2015-01-11

I have to write ms sql query,

Answer 1

I'm assuming this table:

SELECT * 
INTO #Dates
FROM (VALUES
  (CAST('2015-01-01' AS DATE)),
  (CAST('2015-01-02' AS DATE)),
  (CAST('2015-01-03' AS DATE)),
  (CAST('2015-01-06' AS DATE)),
  (CAST('2015-01-07' AS DATE)),
  (CAST('2015-01-11' AS DATE))) dates(d);

Here's a recursive solution with explanations:

WITH
  dates AS (
    SELECT
      d, 
      -- This checks if the current row is the start of a new group by using LAG()
      -- to see if the previous date is adjacent
      CASE datediff(day, d, LAG(d, 1) OVER(ORDER BY d)) 
        WHEN -1 THEN 0 
        ELSE 1 END new_group,
      -- This will be used for recursion
      row_number() OVER(ORDER BY d) rn
    FROM #Dates
  ),
  -- Here, the recursion happens
  groups AS (
    -- We initiate recursion with rows that start new groups, and calculate "GRP"
    -- numbers
    SELECT d, new_group, rn, row_number() OVER(ORDER BY d) grp
    FROM dates
    WHERE new_group = 1
    UNION ALL
    -- We then recurse by the previously calculated "RN" until we hit the next group
    SELECT dates.d, dates.new_group, dates.rn, groups.grp
    FROM dates JOIN groups ON dates.rn = groups.rn + 1
    WHERE dates.new_group != 1
  )
-- Finally, we enumerate rows within each group
SELECT d, row_number() OVER (PARTITION BY grp ORDER BY d)
FROM groups
ORDER BY d

SQLFiddle