常用SQL语句

1. 查看表中字段定义

-- MySQL/Oracle：  
DESC your_table;  

-- 或者通用信息视图：  
SELECT 
  column_name, data_type, character_maximum_length, is_nullable 
FROM information_schema.columns 
WHERE table_schema = 'your_schema' 
  AND table_name   = 'your_table';

2. 查看表的总行数 查看表的前5行

-- 总行数  
SELECT COUNT(*) AS total_rows 
FROM your_table;

-- 前几行样本  
SELECT * 
FROM your_table 
LIMIT 5;

3. 检查col1 和 col2 空值的个数

SELECT
  SUM(CASE WHEN col1 IS NULL THEN 1 ELSE 0 END) AS col1_nulls,
  SUM(CASE WHEN col2 IS NULL THEN 1 ELSE 0 END) AS col2_nulls,
  ... 
FROM your_table;

4. 查看建表语句

SHOW CREATE TABLE your_table_name;