SELECT
hs.student_key,
ss.student_name
FROM hub_students AS hs
JOIN sat_student AS ss ON hs.student_key = ss.student_key;
從模型擷取資料（1）
LEFT JOIN ON：SQL 子句，根據鍵在 ON 條件下，保留左側實體的所有列，並合併右表中相符的列。
SELECT
hs.student_key,
ss.student_name
FROM hub_students AS hs
JOIN sat_student AS ss ON hs.student_key = ss.student_key
LEFT JOIN link_enrollment AS le ON hs.student_key = le.student_key
從模型擷取資料（2）
COUNT：SQL 聚合函式，回傳群組中的項目數量。
GROUP BY：SQL 子句，將具有相同值的資料彙總。
SELECT
hs.student_key,
ss.student_name,
COUNT(le.class_key) AS NumberOfEnrollments
FROM hub_students AS hs
JOIN sat_student AS ss ON hs.student_key = ss.student_key
LEFT JOIN link_enrollment AS le ON hs.student_key = le.student_key
GROUP BY hs.student_key,
ss.student_name
從模型擷取資料（3）
MAX：SQL 聚合函式，找出某屬性的一組值中的最大值。
SELECT
hs.student_key,
ss.student_name,
COUNT(le.class_key) AS NumberOfEnrollments
MAX(sc.load_date) AS MostRecentEnrollmentDate
FROM hub_students hs
JOIN sat_student ss ON hs.student_key = ss.student_key
LEFT JOIN link_enrollment le ON hs.student_key = le.student_key
LEFT JOIN sat_class sc ON le.class_key = sc.class_key
GROUP BY hs.student_key,
ss.student_name;
函式總覽
SELECT FROM：SQL 指令，用於擷取實體中的欄位。
JOIN ON：SQL 子句，依相關屬性合併實體的列。
LEFT JOIN ON：SQL 子句，根據鍵在 ON 條件下，保留左側實體的所有列，並合併右表相符列；若無相符，結果仍顯示左側列，右側屬性為空值。
COUNT：SQL 聚合函式，回傳群組中的項目數量。
MAX：SQL 聚合函式，找出屬性的最大值。
GROUP BY：SQL 子句，將具有相同值的資料彙總。
函式總覽
SELECT column_name,
COUNT(another_column) AS alias_name,
MAX(other_column) AS alias_name
FROM table_name table_alias
-- 依鍵合併實體
JOIN other_table AS other_alias
ON table_alias.FK = other_alias.PK
LEFT JOIN another_table AS another_alias
ON table_alias.FK = other_alias.PK
-- 依特定欄位彙總資料
GROUP BY column_name;
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