Postgres 错误：用作表达式的子查询返回多于一行

处理PostgreSQL错误：用作表达式的子查询返回多于一行

sql
   SELECT column_name
   FROM table_name
   WHERE some_condition
   ORDER BY some_column
   LIMIT 1;
   

sql
   SELECT MAX(column_name)
   FROM table_name
   WHERE some_condition;
   

sql
-- 创建示例表
CREATE TABLE students (
    student_id SERIAL PRIMARY KEY,
    student_name VARCHAR(100)
);
CREATE TABLE grades (
    student_id INT,
    grade INT
);
-- 插入示例数据
INSERT INTO students (student_name) VALUES ('Alice'), ('Bob');
INSERT INTO grades (student_id, grade) VALUES
    (1, 90),
    (1, 95),
    (2, 88),
    (2, 92);
-- 查询每个学生的最高分（错误的查询）
SELECT student_name, (SELECT grade FROM grades WHERE student_id = students.student_id ORDER BY grade DESC) AS max_grade
FROM students;

sql
-- 修复查询
SELECT student_name, (SELECT MAX(grade) FROM grades WHERE student_id = students.student_id) AS max_grade
FROM students;