MySQL function to find the number of working days between two dates

by

This expression –

5 * (DATEDIFF(@E, @S) DIV 7) + MID('0123444401233334012222340111123400012345001234550', 7 * WEEKDAY(@S) + WEEKDAY(@E) + 1, 1)

calculates the number of business days between the start date @S and the end date @E.

Assumes end date (@E) is not before start date (@S).
Compatible with DATEDIFF in that the same start date and end date
gives zero business days.
Ignores holidays.

The string of digits is constructed as follows. Create a table of
start days and end days, the rows must start with monday (WEEKDAY
0) and the columns must start with Monday as well. Fill in the
diagonal from top left to bottom right with all 0 (i.e. there are 0
working days between Monday and Monday, Tuesday and Tuesday, etc.).
For each day start at the diagonal (must always be 0) and fill in
the columns to the right, one day at a time. If you land on a
weekend day (non business day) column, the number of business days
doesn’t change, it is carried from the left. Otherwise, the number
of business days increases by one. When you reach the end of the
row loop back to the start of the same row and continue until you
reach the diagonal again. Then go on to the next row.

E.g. Assuming Saturday and Sunday are not business days –

 | M T W T F S S
-|--------------
M| 0 1 2 3 4 4 4
T| 4 0 1 2 3 3 3
W| 3 4 0 1 2 2 2
T| 2 3 4 0 1 1 1
F| 1 2 3 4 0 0 0
S| 1 2 3 4 5 0 0
S| 1 2 3 4 5 5 0

Then concatenate the 49 values in the table into the string.

Please let me know if you find any bugs.

-Edit
improved table:

 | M T W T F S S
-|--------------
M| 0 1 2 3 4 4 4
T| 4 0 1 2 3 3 3
W| 3 4 0 1 2 2 2
T| 2 3 4 0 1 1 1
F| 1 2 3 4 0 0 0
S| 0 1 2 3 4 0 0
S| 0 1 2 3 4 4 0

improved string: ‘0123444401233334012222340111123400001234000123440’

improved expression:

5 * (DATEDIFF(@E, @S) DIV 7) + MID('0123444401233334012222340111123400001234000123440', 7 * WEEKDAY(@S) + WEEKDAY(@E) + 1, 1)

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