Since interleague play was first introduced way back yonder in 1997, the American League has pretty much owned the National League. For evidence of this stirring claim, here's the year-by-year rundown of interleague results:  

Year Best record AL wins NL wins Win PCT
1997 National 97 117 0.547
1998 American 114 110 0.509
1999 National 116 135 0.538
2000 American 136 115 0.542
2001 American 132 120 0.524
2002 National 123 129 0.512
2003 National 115 137 0.544
2004 American 127 125 0.504
2005 American 136 116 0.54
2006 American 154 98 0.611
2007 American 137 115 0.544
2008 American 149 103 0.591
2009 American 138 114 0.548
2010 American 134 118 0.532
2011 American 131 121 0.52
2012 American 142 110 0.563
2013 American 154 146 0.513
2014 American 163 137 0.543
2015 American 167 133 0.557
2016 American 165 135 0.55

As you can see, the AL have prevailed in interleague play for 13 straight seasons. Overall -- i.e., since 1997 and not counting World Series play and All-Star Games -- the AL has a record of 2,730-2,434 in interleague play, which comes to a .529 winning percentage. Over that same span, they've outscored the NL in those head-to-head games by 1,714 runs. 

And what about those other interleague encounters? In World Series games from 1997 through 2016, the AL champ has gone 60-48 against its NL counterpart. In the All-Star Game (counting this year's), the AL has gone a whopping 17-3-1 since '97. Add it all up, and since the dawn of interleague play, the AL coming into this season was 321 games over .500 against the NL in interleague play, World Series games and All-Star games (counting the 2017 All-Star Game).

As for whether this means the AL has been the superior league over the span in question, the author will leave that to you. What's beyond dispute is that the AL has owned interleague play, especially from Y2K onward. Sure, you can argue that some scheduling inequities have been at work to an extent (we had regional pairing for the first five years of interleague play and rotating arrangements since then), but even if that breaks in the NL's favor it doesn't explain away the margins involved. 

Hills be shaken: There's hope for the NL rooter in 2017. Going into Wednesday's slate, the AL has a 125-120 record against the NL in interleague play this season. That's a lead, yes, but it's a manageable one. Also, keep in mind that interleague contests now pepper the MLB schedule from start to finish these days. Indeed, we've still got more than 50 interleague games still on the schedule, and roughly half of those will involve an NL team that's presently in playoff position. So the NL may yet put its best interleague foot forward in 2017. Given the tight head-to-head record, we've got a shot at our best interleague "finish" since 2004 and a shot at the first NL triumph since 2003. 

Obviously, interleague results through the prism of league superiority is of less pressing importance right now than the playoff races and even August trade speculation. It is, however, an interesting subplot as we head toward the 2017 stretch drive, as the senior circuit has an opportunity before it that it hasn't enjoyed in a very long time.