Subquery with more than one result column not valid.
说明：具有多个子查询结果列无效。
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.
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。
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。
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。
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