MariaDB の SQL Statements & Structure における ROW_NUMBER

MariaDB の ROW_NUMBER 関数：詳細解説

ROW_NUMBER 関数の利点:

• 各行に 一意の識別番号 を提供します。
• 結果の順序付け を簡潔に行うことができます。
• 部分集の合計 や 平均値 を簡単に計算できます。
• 複雑なクエリ をより簡単に記述できます。

ROW_NUMBER 関数の構文:

ROW_NUMBER() OVER ( [PARTITION BY partition_expression] ORDER BY order_expression )

構文の詳細:

• PARTITION BY 句: 結果をグループ化するための式を指定します。
• ORDER BY 句: シーケンス番号を割り当てる順序を指定します。

例:

全ての行にシーケンス番号を割り当てる:

SELECT ROW_NUMBER() OVER () AS row_num, * FROM table;

部門別にシーケンス番号を割り当てる:

SELECT ROW_NUMBER() OVER (PARTITION BY department_id ORDER BY salary DESC) AS row_num, * FROM employees;

各部門の上位 3 人の従業員を取得する:

SELECT * FROM (
  SELECT ROW_NUMBER() OVER (PARTITION BY department_id ORDER BY salary DESC) AS row_num, * FROM employees
) AS t WHERE row_num <= 3;

その他のウィンドウ関数:

• RANK()
• DENSE_RANK()
• PERCENT_RANK()
• NTILE()
• LEAD()
• LAG()

補足:

• ROW_NUMBER 関数は、MariaDB 10.2 以降で使用できます。
• ROW_NUMBER 関数は、他のウィンドウ関数と組み合わせて使用できます。

ROW_NUMBER 関数のサンプルコード

SELECT ROW_NUMBER() OVER () AS row_num, * FROM employees;

# 結果

| row_num | id | name        | department | salary |
|---------|----|-------------|------------|--------|
| 1       | 1  | John Doe     | Sales      | 100000 |
| 2       | 2  | Jane Doe     | Marketing  | 80000  |
| 3       | 3  | Peter Smith  | IT         | 90000  |
| 4       | 4  | Sarah Jones  | HR         | 70000  |

部門別にシーケンス番号を割り当てる:

SELECT ROW_NUMBER() OVER (PARTITION BY department ORDER BY salary DESC) AS row_num, * FROM employees;

# 結果

| row_num | id | name        | department | salary |
|---------|----|-------------|------------|--------|
| 1       | 3  | Peter Smith  | IT         | 90000  |
| 2       | 1  | John Doe     | Sales      | 100000 |
| 3       | 2  | Jane Doe     | Marketing  | 80000  |
| 4       | 4  | Sarah Jones  | HR         | 70000  |

各部門の上位 3 人の従業員を取得する:

SELECT * FROM (
  SELECT ROW_NUMBER() OVER (PARTITION BY department ORDER BY salary DESC) AS row_num, * FROM employees
) AS t WHERE row_num <= 3;

# 結果

| id | name        | department | salary |
|----|-------------|------------|--------|
| 3  | Peter Smith  | IT         | 90000  |
| 1  | John Doe     | Sales      | 100000 |
| 2  | Jane Doe     | Marketing  | 80000  |

同点の場合の処理:

SELECT ROW_NUMBER() OVER (PARTITION BY department ORDER BY salary DESC) AS row_num, * FROM employees;

# 結果

| row_num | id | name        | department | salary |
|---------|----|-------------|------------|--------|
| 1       | 3  | Peter Smith  | IT         | 90000  |
| 1       | 1  | John Doe     | Sales      | 100000 |
| 3       | 2  | Jane Doe     | Marketing  | 80000  |
| 4       | 4  | Sarah Jones  | HR         | 70000  |

RUNNING TOTAL を計算する:

SELECT ROW_NUMBER() OVER (ORDER BY salary) AS row_num,
       SUM(salary) OVER (ORDER BY salary ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) AS running_total
FROM employees;

# 結果

| row_num | running_total |
|---------|---------------|
| 1       | 70000         |
| 2       | 150000        |
| 3       | 240000        |
| 4       | 330000        |

GAP を計算する:

SELECT ROW_NUMBER() OVER (ORDER BY salary) AS row_num,
       salary - LAG(salary) OVER (ORDER BY salary) AS salary_gap
FROM employees;

# 結果

| row_num | salary_gap |
|---------|------------|
| 1       | NULL        |
| 2       | 30000       |
| 3       | 10000       |
| 4       | 90000       |

NTILE を計算する:

SELECT ROW_NUMBER() OVER (ORDER BY salary) AS row_num,
       NTILE(4) OVER (ORDER BY salary) AS ntile
FROM employees;

# 結果

| row_num | ntile |
|---------|--------|
| 1       | 1      |
| 2       | 1      |

ROW_NUMBER 関数の代替方法

サブクエリ:

SELECT t.row_num, *
FROM (
  SELECT ROW_NUMBER() OVER (ORDER BY salary) AS row_num, *
  FROM employees
) AS t;

JOIN:

SELECT e.id, e.name, e.department, e.salary, r.row_num
FROM employees AS e
JOIN (
  SELECT ROW_NUMBER() OVER (ORDER BY salary) AS row_num, id
  FROM employees
) AS r ON e.id = r.id;

CTE (Common Table Expressions):

WITH t AS (
  SELECT ROW_NUMBER() OVER (ORDER BY salary) AS row_num, *
  FROM employees
)
SELECT * FROM t;

ユーザー変数:

SET @row_num = 0;

SELECT @row_num := @row_num + 1 AS row_num, *
FROM employees
ORDER BY salary;

どの方法を使用するかは、状況によって異なります。 以下は、各方法の利点と欠点です。

サブクエリ:

• 利点: わかりやすい
• 欠点: 複雑なクエリになる場合がある

JOIN:

• 利点: 効率的な場合が多い
• 欠点: 複雑なクエリになる場合がある

CTE:

• 利点: 読みやすい
• 欠点: MariaDB 10.2 以降でのみ使用可能

ユーザー変数:

• 利点: 効率的な場合が多い
• 欠点: 複雑なクエリになる場合がある

ROW_NUMBER 関数は、多くの場合、最も簡単な方法ですが、他の方法も検討することをお勧めします。