問題描述：

有一張表：t_student

字段：
id
name
age
status (0,1,2,3,4,5,6)
teacher_id

如果teacher_id是空，就不查詢status=0
這種SQL語句怎么寫？

解決方法：

WITH T AS (
  SELECT 1 AS ID, 'A' AS NAME, 21 AS AGE, 0 AS STATUS, 1 AS TEACHERID FROM DUAL UNION ALL 
  SELECT 2 AS ID, 'B' AS NAME, 20 AS AGE, 1 AS STATUS, 1 AS TEACHERID FROM DUAL UNION ALL
  SELECT 3 AS ID, 'C' AS NAME, 21 AS AGE, 2 AS STATUS, 2 AS TEACHERID FROM DUAL UNION ALL
  SELECT 4 AS ID, 'D' AS NAME, 22 AS AGE, 3 AS STATUS, 3 AS TEACHERID FROM DUAL UNION ALL
  SELECT 5 AS ID, 'E' AS NAME, 23 AS AGE, 0 AS STATUS, NULL AS TEACHERID FROM DUAL 
)
SELECT * FROM T
WHERE (TEACHERID IS NULL AND STATUS <> 0) OR TEACHERID IS NOT NULL;