MySQL (iii) DQL basic query

1. Grammar

SELECT what to query from the table name;

Similar to Java: System.out.println (something to print);

2. Features

① The result of a select query is a virtual table, not a real existence.

② to query can be a constant value, can be an expression, can be a field, can be a function.

3. Querying a single field in a table

SELECT last_name from Employees;

4. Querying multiple fields in a table

SELECT Last_name,salary,email from Employees;

5. Querying all fields in the table

SELECT * FROM Employees;

6. Query constant Value

SELECT ' John ';

7. Query expression

SELECT 100%98;

8. Query function

SELECT VERSION ();

9, from the alias

① Easy to understand

② If the field you want to query has the same name, you can distinguish it by using an alias

Mode one: Use as

SELECT Last_Name as surname, first_name as name from employees;

Way two: Use spaces

SELECT last_name Surname, first_name name from employees;

Case: Query salary, display results as out put, enclosed in double quotes

Select salary as "out put" from employees;

10, go to Heavy

Case: Querying all the department numbers involved in the employee table

SELECT DISTINCT department_id from employees;

11, + Number of the role

The + sign in Java:

① operator with two operands of numeric type

② connector, as long as one operand is a string

MySQL in the + number:

There is only one function: operator

Select 100+90; Two operands are numeric, then add operation

Select ' 123 ' +90; if one of the characters is a character type, try to convert the character value into a numeric type, and if the conversion succeeds, continue with the addition operation

Select ' John ' +90; Converts a character value to 0 if the conversion fails

Select Null+10; As long as one of the two is null, the result must be null

12, CONCAT

Case: Query employee name and last name concatenated into a field and displayed as name

SELECT CONCAT (last_name,first_name) as name from employees;

If you concatenate one more empty field, the result is null

Workaround, use the Ifnull function

MySQL (iii) DQL basic query