WHERE句と SELECT句内の サブクエリ

SQLのデータ結合

Maham Faisal Khan

Senior Content Developer, DataCamp

WHERE内のサブクエリ構文

  • これまでのセミ結合とアンチ結合は、WHERE にサブクエリを使用
  • WHERE内はサブクエリが最も使われる場所

WHERE IN句を使ったクエリ構文

SELECT *
FROM some_table
WHERE some_numeric_field IN (4, 8, 12);
SQLのデータ結合

WHERE内のサブクエリ構文

SELECT *
FROM some_table
WHERE some_field IN 
       (include subquery here);
SQLのデータ結合

WHERE内のサブクエリ構文

SELECT *
FROM some_table
WHERE some_field IN 
    (SELECT some_numeric_field
     FROM another_table
     WHERE field2 = some_condition);
SQLのデータ結合

SELECT句内のサブクエリ

SELECT DISTINCT continent
FROM states;

|---------------|
| continent     |
|---------------|
| Africa        |
| Asia          |
| Europe        |
| North America |
| Oceania       |
|---------------|
SQLのデータ結合

SELECT句内のサブクエリ

SELECT DISTINCT continent,

    (SELECT COUNT(*)
     FROM monarchs

     WHERE states.continent = monarch.continent) AS monarch_count
FROM states;

| continent     |   monarch_count |
|---------------|-----------------|
| Africa        |               0 |
| Asia          |               2 |
| Europe        |               2 |
| North America |               0 |
| Oceania       |               0 |
SQLのデータ結合

練習しましょう！

SQLのデータ結合

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