SQL语句之-计算字段/分组

五、计算字段

1、拼接字段

MySQL：使用函数concat

SqlServer：使用加号+

oracle：使用||

SELECT CONCAT(vend_name,'(',vend_country,')') FROM vendors ORDER BY vend_name

使用别名AS

SELECT CONCAT(vend_name,'(',vend_country,')')  AS vend_title FROM vendors ORDER BY vend_name

2、算数计算：支持+、—、*、/，用（）区分优先级

SELECT prod_id,quantity,item_price,
quantity*item_price AS expanded_price
FROM orderitems WHERE order_num = 20008

 

六、分组

1、过滤分组

WHERE过滤行，而HAVING过滤分组。

SELECT vend_id, COUNT(*) AS num_prods FROM products GROUP BY vend_id;
SELECT cust_id,COUNT(*) AS num_orders FROM orders GROUP BY cust_id HAVING COUNT(*)>=2;

SELECT vend_id,prod_price,COUNT(*) AS num_prods FROM products WHERE prod_price >= 4 GROUP BY vend_id HAVING COUNT(*)>= 2;//结合where语句

SELECT order_num,COUNT(*) AS items FROM orderitems GROUP BY order_num HAVING COUNT(*)>=3 ORDER BY items//分组没有排序，必须用order by指定排序