非常好的外国SQL教程！！强烈推荐
Introduction
Week 1 at a Glance
Day 1 Introduction to SQL
Day 2 Introduction to the Query: The SELECT Statement
Day 3 Expressions, Conditions, and Operators
Day 4 Functions: Molding the Data You Retrieve
Day 5 Clauses in SQL
Day 6 Joining Tables
Day 7 Subqueries: The Embedded SELECT Statement
Week 1 in Review
Week 2 at a Glance
Day 8 Manipulating Data
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