插图:

mtwtfSSmtwtfSS
  123456712345   one week plus 5 days, you can remove whole weeks safely
  12345-------   you can analyze partial week's days at start date
  -------12345   or at ( end date - partial days )

伪代码:

@S          = start date
@E          = end date, not inclusive
@full_weeks = floor( ( @E-@S ) / 7)
@days       = (@E-@S) - @full_weeks*7   OR (@E-@S) % 7

SELECT
  @full_weeks*5 -- not saturday+sunday
 +IF( @days >= 1 AND weekday( S+0 )<=4, 1, 0 )
 +IF( @days >= 2 AND weekday( S+1 )<=4, 1, 0 )
 +IF( @days >= 3 AND weekday( S+2 )<=4, 1, 0 )
 +IF( @days >= 4 AND weekday( S+3 )<=4, 1, 0 )
 +IF( @days >= 5 AND weekday( S+4 )<=4, 1, 0 )
 +IF( @days >= 6 AND weekday( S+5 )<=4, 1, 0 )
 -- days always less than 7 days