这句Update为什么有错误？

[tatty_bad]

Update [TableB] a inner join [TableC] b on a.a1 = b.a1 and a.a4 = b.a4 and a.a7 = b.a7 and a.a11 = b.a11 and b.AID = 1 set a.a17 = b.a17,a.a18 = b.a18,a.a19 = b.a19,a.a20 = b.a20

在查询分析器里面，提示是 a 附近有错误！