Postgres：将单行转换为多行（unpivot）

使用PostgreSQL进行单行转多行（Unpivot）操作

sql
CREATE TABLE student_grades (
    student_id INT,
    math_grade INT,
    science_grade INT,
    history_grade INT
);
INSERT INTO student_grades VALUES
    (1, 85, 90, 78),
    (2, 92, 88, 95),
    (3, 78, 85, 80);

sql
SELECT student_id, 'math' AS subject, math_grade AS grade FROM student_grades
UNION ALL
SELECT student_id, 'science' AS subject, science_grade AS grade FROM student_grades
UNION ALL
SELECT student_id, 'history' AS subject, history_grade AS grade FROM student_grades;