Why Was a New Query Language Needed?

Historically, the work that led to the development of XQuery started long before XSLT 1.0 and XPath 1.0 were published. At first there was little contact between the two groups. During 1998 and 1999 there was some cross-influence between XQL, one of the precursors of XQuery, and the emerging XPath language (probably each language influenced the other, though this is hard to verify). But neither group would have seen the other language as being directly relevant to the requirements they were addressing—the degree of overlap only became apparent later.

The differences between XSLT and XQuery are of two kinds. First, they have different requirements, and therefore a design decision that was appropriate for XSLT would not necessarily be right for XQuery, and vice versa. The second kind of difference results from their being designed by different people from different communities and computing traditions, with different beliefs about what constitutes good design, and different experiences as to what works well and what doesn't.