Alterna · учебный материал
Database requests
Selection, sorting and selection of fields to answer a specific question.
Что в этом материале
- Request - Data Question
- Selection
- Sorting
- Fields
- Check the condition.
- Parameter and computable field
- Parametric request
- Computed field
Содержание и основные понятия
01
Request - Data Question
Selection
"Show the books of the "Scientific" category that are available.
Sorting
"Strike the year's result from the new to the old."
Fields
"Show me only a name, a year, and a status, not everything."
02
Check the condition.
?
Read Request
The condition "category = Science and status = in existence" requires that two conditions be fulfilled simultaneously. The request "category=science or status = available" will return more lines.
03
Parameter and computable field
Parametric request
The value of the condition shall be entered at start-up: category, starting date or minimum price. The parameter type shall be set in advance so that the line is not compared to the number by accident.
Computed field
It's a formula from the fields of recording, for example.
Стоимость = Цена × Количество
It is not duplicated in the table if it is clearly restated.
Composite request:
Pick fields, connect key tables, make a condition.
И/ИЛИ/НЕ
I want you to compute the right field, sort out the result, and then draw a conclusion.