SQL connection query, variable, operator, branch, Loop statement

Connection query: You can implement multiple table queries by using the Join operator. Connection is the main feature of relational database model, and it is also a sign that distinguishes it from other types of database management system.

Common two link operators:

1.join on

2.union

In the relational database management system, the relationship between the data is not determined, and all the information of an entity is often stored in a single table. When retrieving data, the connection operation queries the information for different entities that reside in multiple tables. The connection operation gives the user a lot of flexibility and they can add new data types at any time. Create a new table for different entities, and then query through the connection.

Example one:

Example two:

Example three:

Example four:

Example five:

Example SIX:

Example Seven:

Example eight:

Variable:

The SQL language, like other programming languages, has control statements such as variables, branches, loops, and so on.

In SQL language, variables are divided into local variables and global variables, and global variables are called system variables.

Local variables:

Using the DECLARE keyword to declare a variable, the syntax is simple: DECLARE @< variable name > < variable type >

You can assign values to variables using the Set keyword, and you can assign only one value to a variable at a time using the Set keyword.

We can also assign a value to this variable in a query statement.

Example:

Global variables: Also called system variables.

Examples of several system variables that are commonly used:

Operator:

Operator Precedence:

If...else ... Statement:

Example:

While statement:

Example:

While if nested example:

Example of break in while:

Example of parsing each step of a loop print:

Case ... when ... Example:

SQL connection query, variable, operator, branch, Loop statement