mysql - * 和 的含义。在MySql中

Question

我不明白此查询中 e.*、at_firstname.value 的用法:

SELECT e.*
     , at_firstname.value AS firstname 
  FROM customer_entity AS e 
 INNER 
  JOIN customer_entity_varchar AS at_firstname 
    ON (at_firstname.entity_id = e.entity_id) 
   AND (at_firstname.attribute_id = '5') 
 WHERE (e.entity_type_id = '1') 
   AND (at_firstname.value = 'sander') 
 ORDER 
    BY e.email ASC 

Answer 1

你的意思可能是

 SELECT e.*, at_firstname.value AS firstname 
  FROM  `customer_entity` AS `e`
  INNER JOIN at_firstname   ON
    at_firstname.entity_id = e.entity_id AND at_firstname.attribute_id = '5'
  WHERE e.entity_type_id = '1' AND at_firstname.value = 'sander' ORDER BY e.email ASC 

e 是 customer_entity 表的别名 (customer_entity AS e )，因此 e.* 表示 customer_entity 表的所有列。

at_firstname.value 似乎是一个错误，因为连接条件只需要表名而不需要 table.column

无论如何，sintax at_firstname.value AS firstname 意味着 tablename.columnname 使用列的别名 firtsname 显示

否则，如果原始查询正确，则 at_firstname.value AS 名字是数据库at_firstname和表值的表别名at_firstname(列名值)的别名

PS 在不需要时不要在条件周围使用无用的 ()