SQL：如何在查询中用例和转换

使用SQL中的CASE语句和转换函数进行查询

sql
SELECT
  column1,
  column2,
  CASE
    WHEN condition1 THEN result1
    WHEN condition2 THEN result2
    ELSE result3
  END AS new_column
FROM
  your_table;

sql
SELECT
  employee_id,
  first_name,
  last_name,
  salary,
  CASE
    WHEN salary > 50000 THEN 'High Salary'
    WHEN salary > 30000 THEN 'Medium Salary'
    ELSE 'Low Salary'
  END AS salary_category
FROM
  employees;

sql
SELECT
  order_id,
  order_date,
  CAST(order_total AS DECIMAL(10, 2)) AS formatted_total
FROM
  orders;

sql
SELECT
  student_id,
  CONCAT(first_name, ' ', last_name) AS student_name,
  exam_score,
  CASE
    WHEN exam_score >= 90 THEN 'Excellent'
    WHEN exam_score >= 70 THEN 'Good'
    WHEN exam_score >= 50 THEN 'Pass'
    ELSE 'Fail'
  END AS score_category,
  CAST(exam_score AS DECIMAL(5, 2)) AS formatted_score
FROM
  students;