Database development Basics-sql Server primary key, foreign key, subquery (nested query)

Primary key

a database primary key is a combination of one column or column in a table whose value uniquely identifies each row in the table. Such a column or columns is called the table's primary key, which enforces the entity integrity of the table. When you create or change a table, you can define PRIMARY key constraint to create a primary key. A table can have only one  PRIMARY KEY constraints, and PRIMARY KEY columns in the constraint cannot accept null values. Because PRIMARY KEY constraints ensure unique data, so it is often used to define identity columns.

the role of the primary key

The main functions of the primary key are as follows:

(1) Ensure the integrity of the entity;

(2) speed up the operation of the database;

(3) When a new record is added to a table, the database automatically checks the primary key value of the new record and does not allow the value to be duplicated with the primary key value of other records;

(4) The database automatically displays the records in the table in the order of the primary key values. If no primary key is defined, records in the table are displayed in the order in which they are entered.

The primary key features: Uniqueness, non-nullability.

Example of setting a primary KEY statement:

Code int PRIMARY KEY, primary key cannot be empty, cannot be duplicated, ensures uniqueness

Example of setting a self-growing primary KEY statement:

The code int primary key identity (1) starts at 1 each time , adding values you do not need to add this column

To set a foreign key:

Right-click on the table where you want to set the foreign key, select design, and right-click the column name where you want to set the foreign key, such as:

Select Relationship Click, the dialog box appears, click Add, click the ellipsis following the table and column specification, such as:

In the interface that appears, do the following:

Click OK, then click OK, the operation is successful.

Sub-query, also called nested query.

make a query statement as a result set for other SQL statements are used, as with normal tables, as query statements that are treated as result sets are called sub-queries.

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

Database development Basics-sql Server primary key, foreign key, subquery (nested query)