SQL primary, foreign key, subquery

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 create a primary key by defining the PRIMARY key constraint. A table can have only one PRIMARY key constraint, and a column in the PRIMARY key constraint cannot accept null values. Because the PRIMARY KEY constraint ensures unique data, 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:

Code int primary key identity (1), 1 increase each time, add values without adding 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.

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.

Practice

Create two tables: 1 Optional Course ID account name Teacher name Teacher Age 2   The name of the student selected class ID I'm going  to choose a teacher  to teach the Class I will choose the teacher the youngest class which  class the students choose which  teacher  How many years of age have several candidates  teacher a   What do you call it  ?  add an Age column to a student's list I'm going to teach teacher A's students the youngest of all students who choose Math   Student information for all students who choose a teacher older than 20

1 Create TableStudent2 (3Scodeint Primary Key Identity(1001,1),4Snamevarchar(Ten),5Xuankeint6 )7 Create Tablewww8 (9Tcodeint Primary Key Identity(1,1),TenKemuvarchar( -), OneTnamevarchar(Ten), AAgeint - ) - Insert  intoWwwValues('Mathematics','Zhang San', to) the Insert  intoStudentValues('AA',1) - Select *  fromStudent - Select *  fromwww - SelectKemu fromWwwwhereTname='Zhang San' + Select Top 1Kemu fromWwwOrder  by Age - SelectKemu,tname,age fromWwwwhereTcode=(SelectXuanke fromStudentwhereScode=1002) + SelectSname fromStudentwhereXuanke=(SelectTcode fromWwwwhereTname='Zhang San') A Alter TableStudent at AddSageint - Select Top 1Sname fromStudentwhereXuanke=(SelectTcode fromWwwwhereTname='Zhang San')Order  bySage - Select *  fromWwwwhereAge<(SelectSage fromStudentwhereScode=1003) - Select *  fromStudentwhereXuankeinch(SelectTcode fromWwwwhereKemu='Mathematics') - Select *  fromStudentwhereXuankeinch(SelectTcode fromWwwwhereAge> -)

SQL primary, foreign key, subquery