Subqueries are not allowed in this context.
在此上下文中不允许使用子查询。
There are two types of subqueries, uncorrelated and correlated.
有两种类型的子查询： 不相关的和相关的。
Subqueries are not allowed in this context. Only scalar expressions are allowed.
在此上下文中不允许使用子查询。只允许使用标量表达式。
You can specify this clause in any top-level SELECT statement and in most types of subqueries .
你可以在任何一个顶层的SELECT语句以及几乎所有类型的子查询语句前，使用子查询定义子句。
Moreover, they should avoid the complex forms of nested and correlated subqueries that make current SQL hard to master.
此外，他们应该避免复杂表格嵌套和相关的SQL语句，使目前难以掌握。
This tutorial describes subqueries stated as conditions in the WHERE clause, which are useful for simpler comparison queries.
本教程描述在 WHERE 子句中作为条件的子查询，这些子查询对于较简单的比较查询很有用。
Syntactically, subqueries are enclosed within parentheses. For example, the following SELECT statement contains a simple subquery in its WHERE clause
从语法上来说,子查询被包含在圆括号中.
Subqueries are generally used to return a single row as an atomic value although they may be used to compare values against multiple rows with the IN keyword.
子查询一般用来返回一个单独的行作为原子值，然而还可以用来比较多行的值，利用IN关键字。
Correlated subqueries will greatly degrade the speed of data retrieval, especially for non-selective queries on huge tables with millions of rows and with no index.
相关子查询将极大地降低数据检索的速度，对于拥有几百万个行但没有索引的大型表的非选择性查询更是如此。
Correlated subqueries will greatly degrade the speed of data retrieval, especially for non-selective queries on huge tables with millions of rows and with no index.
相关子查询将极大地降低数据检索的速度，对于拥有几百万个行但没有索引的大型表的非选择性查询更是如此。
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