The second major function performed by a WHERE clause is to join multiple tables together into a single table that can be queried more easily.
WHERE 子句执行的第二个主要功能是将多个表连接到单个表中，从而更易于查询。
Before you can start with the actual analysis, you need to combine the four tables into a single table. This single combined table should contain the following columns.
在您可以开始实际分析以前，您需要把这四个表合并成一个单一的表。
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