• Rearranged position of some figures.

• Fixed horrible line breaking in math formula. ...

• Corrected annotations of copied projection edge between \TT{NLS} and \TSSNL.
• Added note about whether copied projection edges actually still count as projection edges.

• Switched to \setminus for set differences. ...

• Made proper use of \TrivialSelection and \TrivialProjection.
• Deleted sentence explaining interpretation of edge annotations with respect to reading direction.
• Deleted paragraph containing Batini et al's transformation terminology; not important.
• Clarified what isomorphism means.
• Rewrote transformation discussion to make it clear that the SIGs are being transformed rather than the schemas (even though we are effectively transforming the schemas).
• Clarified composition transformations.
• Various minor phrasing and grammatical changes.

• Removed confusing “clarification” of edge annotations. ...

• Removed footnote about nulls, as it didn’t seem to make sense and added nothing useful.
• Edited discussion of selection and projection edges.

• Various changes to phrasing up to Section 4.1.

• Corrected publication year of Date’s view updating book.

• Updated Section 7.

• Updated Section 6.

• Updated section 5.

• Various edits to Sections 1–4. ...

• Oriented focus more towards modelling.

• Updated to use ACM style.

• Added missing ACM copyright style file.

• Renamed ACM class file back to original name.

• Added initial version of paper.