将主查询A的数据，放到子查询B中做条件验证，根据验证结果（true或false）来决定主查询的数据是否保留

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PostgreSQL 使用 DROP TABLE 语句来删除表格，包含表格数据、规则、触发器等，所以删除表格要慎重，删除后所有信息就消失了。

PostgreSQL 使用 DROP TABLE 语句来删除表格，包含表格数据、规则、触发器等，所以删除表格要慎重，删除后所有信息就消失了。

Suppose that a website contains two tables, the Customers table and the Orders table. Write a SQL query to find all customers who never order anything.

The Employee table holds all employees. Every employee has an Id, a salary, and there is also a column for the department Id.

Write a SQL query for a report that provides the following information for each person in the Person table, regardless if there is an address for each of those ...

Write a SQL query to get the second highest salary from the Employee table.

Write a SQL query to get the nth highest salary from the Employee table.

Write a SQL query to find all numbers that appear at least three times consecutively.