本文探讨了SQL中子查询的正确用法，指出主查询可以包含多个子查询，并且子查询可以从不同表中检索数据，同时支持多列或多表达式的比较。
Q31. Which three statements are true reading subquenes?
A. A Main query can have many subqueries.
B. A subquery can have more than one main query
C. The subquery and main query must retrieve date from the same table.
D. The subquery and main query can retrieve data from different tables.
E. Only one column or expression can be compared between the subquery and main query.
F. Multiple columns or expressions can be compared between the subquery and main query.
Answer: A,D,F
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