Fundamentals of database Development-Lesson Plans-6 sub-queries

Sub-query, also called nested query.

A query statement is used as a result set for other SQL statements, just as a query statement that is treated as a result set is called a subquery, as with a normal table.

There are two types of sub-queries:

One is to return only a single-valued subquery, at which point it can be used where a single value can be used, when a subquery can be considered a function that has a return value;

Another is a subquery that returns a column of values, at which point the subquery can be thought of as a data table that is temporarily present in memory.

Create a new department table, an employee table, and a department column number for each person in the employee table.

Sub-query Example:

Practice One:

Exercise two:

Exercise Three:

Example of a paged query statement:

See how many pages can be divided into a total:

Practice:

Subquery Practice One: Find out All information about people who are older than seven years old in the sales department

Exercise two: convert all numeric codes from a department in a haha table to a string display in a bumen table

Exercise three: Convert all the number codes from the department in the haha table to The string display in the Bumen table, and show each person's supervisor

Fundamentals of database Development-Lesson Plans-6 sub-queries