该查询表达式是一个构建块，可以通过组装它为查询定义最后的选择谓词。
The query expression is a building block that can be assembled to define the final selection predicate for the query.
从现在起，我们把示例中的定义和表达式写在一块，你把它们怎么放都行。
From now on, we'll write our example definitions and expressions together, and you can put them in whichever area you prefer.
这些简单的案例中提供了表达式，但它们也可以是规定多条语句的代码块(使用花括号)。
In these simple cases, expressions are provided, but they could also be blocks of code (using curly braces) to specify multiple statements.
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