SQLiteで計算列を使いこなす！データの冗長性を減らし、クエリをシンプルにする方法

2024-06-22

SQLite で計算列を作成する方法

データの冗長性を減らすことができます。
クエリをよりシンプルにすることができます。
計算結果を常に最新の状態に保つことができます。

計算列の種類

SQLite には、2 種類の計算列があります。

仮想列: 仮想列は、データファイルに格納されません。その値は、毎回クエリが実行されるたびに計算されます。

計算列の作成

計算列は、次の SQL ステートメントを使用して作成できます。

CREATE TABLE table_name (
  column1 datatype,
  column2 datatype,
  ...,
  generated_column_name datatype GENERATED ALWAYS AS (expression) [VIRTUAL | STORED]
);

ここで、

table_name は、テーブルの名前です。
column1, column2 は、テーブルの他の列です。
generated_column_name は、計算列の名前です。
datatype は、計算列のデータ型です。
expression は、計算列の値を計算する式です。
VIRTUAL キーワードは、列が仮想列であることを指定します。

例:

次の SQL ステートメントは、customers テーブルに total_price という名前の計算列を作成します。この列は、price 列と quantity 列の値を掛け合わせた値を格納します。

CREATE TABLE customers (
  customer_id INTEGER PRIMARY KEY,
  name TEXT,
  price REAL,
  quantity INTEGER,
  total_price REAL GENERATED ALWAYS AS (price * quantity) STORED
);

計算列の使用

計算列は、他の列と同じように使用できます。ただし、計算列の値は直接更新することはできません。計算列の値を変更するには、その計算に使用される列の値を変更する必要があります。

次の SQL ステートメントは、customers テーブルのすべての顧客の合計金額を選択します。

SELECT customer_id, name, total_price FROM customers;

注意事項

計算列は、主キーまたはユニーク制約として使用できません。
計算列は、インデックスを作成できません。

SQLite の計算列は、テーブルにデータを格納する方法をより柔軟にする強力な機能です。計算列を使用すると、データの冗長性を減らし、クエリをよりシンプルにし、計算結果を常に最新の状態に保つことができます。

Creating a calculated column

CREATE TABLE orders (
  order_id INTEGER PRIMARY KEY,
  customer_id INTEGER,
  product_id INTEGER,
  quantity INTEGER,
  price REAL,
  total_price REAL GENERATED ALWAYS AS (price * quantity) STORED
);

This code creates a table named orders with the following columns:

order_id: An integer that uniquely identifies the order
customer_id: An integer that references the customer who placed the order
price: A real number that represents the price of the product
total_price: A calculated column that represents the total price of the order, calculated by multiplying the price and quantity columns

The total_price column is a calculated column, which means that its value is automatically calculated from the values of the price and quantity columns. The STORED keyword indicates that the total_price column is a stored column, which means that its values are stored in the database file.

Using a calculated column in a query

SELECT order_id, customer_id, product_id, quantity, price, total_price
FROM orders;

This code selects all of the columns from the orders table, including the total_price calculated column.

Updating a record that uses a calculated column

UPDATE orders
SET customer_id = 123,
    product_id = 456,
    quantity = 10
WHERE order_id = 1;

This code updates the customer_id, product_id, and quantity columns for the order with the ID of 1. The total_price column will automatically be updated to reflect the new values of the price and quantity columns.

I hope these examples help you understand how to use calculated columns in SQLite.

Triggers are a powerful mechanism for automating tasks in SQLite. You can use a trigger to update a calculated column whenever the values of the columns that it depends on are changed.

For example, you could create a trigger that updates the total_price column in the orders table whenever the price or quantity columns are updated:

CREATE TRIGGER update_total_price
AFTER UPDATE ON orders
FOR EACH ROW
BEGIN
  UPDATE orders
  SET total_price = NEW.price * NEW.quantity
  WHERE order_id = NEW.order_id;
END;

This trigger will fire whenever a record in the orders table is updated. The NEW keyword refers to the new values of the columns that were updated.

Views are a way of creating virtual tables based on one or more existing tables. You can create a view that includes a calculated column that is based on the columns of the underlying tables.

For example, you could create a view named orders_with_total_price that includes the total_price calculated column:

CREATE VIEW orders_with_total_price AS
SELECT order_id, customer_id, product_id, quantity, price, (price * quantity) AS total_price
FROM orders;

You can then query the orders_with_total_price view just like you would query any other table.

Materialized views are a special type of view that stores the results of the view's query. This means that the data in the materialized view is always up-to-date, even if the underlying tables are changed.

CREATE MATERIALIZED VIEW orders_with_total_price_materialized AS
SELECT order_id, customer_id, product_id, quantity, price, (price * quantity) AS total_price
FROM orders;

Derived tables are temporary tables that are created from the results of a subquery. You can use a derived table to calculate a column that is based on the columns of other tables.

SELECT order_id, customer_id, product_id, quantity, price, (price * quantity) AS total_price
FROM orders;

You can then use this derived table in any query where you would use a regular table.

Application logic

You can also calculate the values of a calculated column in your application logic. This can be a good option if you need more control over how the values are calculated, or if you need to use the values in calculations that are not possible in SQL.

def calculate_total_price(order):
  total_price = order['price'] * order['quantity']
  return total_price

order = {
  'order_id': 1,
  'customer_id': 123,
  'product_id': 456,
  'quantity': 10,
  'price': 5.99
}

total_price = calculate_total_price(order)
print(total_price)

The best approach for you will depend on your specific needs and requirements. If you need a simple way to calculate a column that is based on other columns in the same table, then calculated columns are a good option. If you need more flexibility or control over how the values are calculated, then one of the other alternatives may be a better choice.

I hope this helps!

sqlite calculated-columns