When this statement is executed, the subquery is evaluated first.
当这条语句执行的时候,子查询会先执行.
It's important to note that technically, this is not a join, but a subquery.
重要的是要注意在技术上这不是一个联接，而是一个子查询。
A. a single row subquery can retrieve data from only one table.
单行子查询可以只从一个表中检索数据。
Subquery with more than one result column not valid.
说明：具有多个子查询结果列无效。
Subquery with more than one result column not valid.
说明：具有多个子查询结果列无效。
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