効率的なデータ管理: SQLiteで自己参照結合と相関副問い合わせを使って列を更新
SQLite テーブル内の列を別の列の値で更新する方法
方法1：UPDATE文と副問い合わせを使用する
この方法は、更新対象となるレコードを明示的に指定する必要がなく、柔軟性が高いのが特徴です。
UPDATE table_name
SET update_column = (
SELECT value_column
FROM table_name
WHERE condition
)
WHERE another_condition;
例:
あるテーブル customers に、顧客 ID、名前、メールアドレスが格納されているとします。このテーブルにおいて、顧客名の末尾に "@example.com" を付加したメールアドレスを email 列に更新したい場合は、以下のクエリを実行します。
UPDATE customers
SET email = (
SELECT name || '@example.com'
FROM customers
WHERE customer_id = 1
)
WHERE customer_id = 1;
このクエリは、customer_id が 1 のレコードの name 列の値に "@example.com" を連結し、その結果を email 列に更新します。
方法2：WITH句を使用する
この方法は、副問い合わせよりも簡潔に記述できる場合があり、可読性が高いのが特徴です。
WITH temp_table AS (
SELECT customer_id, name || '@example.com' AS email
FROM customers
)
UPDATE customers
SET email = t.email
FROM temp_table AS t
WHERE customers.customer_id = t.customer_id;
上記の例と同様に、顧客名の末尾に "@example.com" を付加したメールアドレスを email 列に更新する場合、以下のクエリを実行します。
WITH temp_table AS (
SELECT customer_id, name || '@example.com' AS email
FROM customers
)
UPDATE customers
SET email = t.email
FROM temp_table AS t
WHERE customers.customer_id = t.customer_id;
このクエリは、WITH 句を使用して中間テーブル temp_table を作成し、そのテーブルの email 列の値を customers テーブルの email 列に更新します。
- 上記の例では、
customer_id列を基にレコードを更新していますが、他の列を条件として使用することもできます。 - 更新対象となるレコードを絞り込むために、
WHERE句を追加することができます。 - 複数の列を同時に更新することもできます。
In this example, we will update the price column of the products table to be 10% higher than the current value of the cost column for all products where the category is 'electronics'.
UPDATE products
SET price = cost * 1.1
WHERE category = 'electronics';
Example 2: Updating a column using a subquery to calculate the new value
In this example, we will update the discount column of the orders table to be 5% of the total order amount for each order. The total order amount is calculated by summing the price * quantity for all items in the order.
UPDATE orders
SET discount = (
SELECT SUM(price * quantity)
FROM order_items
WHERE order_id = orders.order_id
) * 0.05
WHERE orders.order_id = 1;
Example 3: Updating multiple columns using a WITH clause
In this example, we will update the last_name and email columns of the customers table based on the values in the first_name and email_domain columns, respectively.
WITH temp_data AS (
SELECT customer_id,
first_name || ' ' || last_name AS full_name,
first_name || '@' || email_domain AS new_email
FROM customers
)
UPDATE customers
SET last_name = t.full_name,
email = t.new_email
FROM temp_data AS t
WHERE customers.customer_id = t.customer_id;
A self-join can be used to update a column in a table based on the value of another column in the same table. This method is particularly useful when you need to update a record based on the value of another record in the same table.
UPDATE t1
SET update_column = t2.value_column
FROM table_name AS t1
JOIN table_name AS t2
ON t1.id = t2.id
WHERE condition;
Example:
In this example, we will update the email column of the customers table to be the same as the email column of the users table for customers who have a matching user_id.
UPDATE customers AS c
SET c.email = u.email
FROM customers AS c
JOIN users AS u
ON c.user_id = u.user_id;
Method 2: Using a correlated subquery
A correlated subquery can be used to update a column in a table based on the value of another column in the same table. This method is similar to using a self-join, but it can be more efficient for large tables.
UPDATE table_name
SET update_column = (
SELECT value_column
FROM table_name
WHERE condition
AND id = table_name.id
);
This example is the same as the first example in the previous section, but using a correlated subquery instead of a self-join.
UPDATE customers
SET email = (
SELECT name || '@example.com'
FROM customers
WHERE customer_id = 1
)
WHERE customer_id = 1;
Method 3: Using a temporary table
A temporary table can be used to store intermediate results and simplify the update query. This method can be useful for complex updates that involve multiple tables or subqueries.
CREATE TEMPORARY TABLE temp_table AS (
SELECT * FROM table_name
WHERE condition
);
UPDATE table_name
SET update_column = temp_table.value_column
FROM table_name
JOIN temp_table ON table_name.id = temp_table.id;
DROP TEMPORARY TABLE temp_table;
This example is the same as the second example in the previous section, but using a temporary table to store the calculated email addresses.
CREATE TEMPORARY TABLE temp_emails AS (
SELECT customer_id, name || '@example.com' AS email
FROM customers
);
UPDATE customers
SET email = temp_emails.email
FROM customers
JOIN temp_emails ON customers.customer_id = temp_emails.customer_id;
DROP TEMPORARY TABLE temp_emails;
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