mysql - 解释子查询和父查询

Question

子查询将在父查询之前执行，子查询将结果传递给父查询....但在下面的父查询中使用子查询结果......

 $sth = $conn->query(
    "SELECT tab.AttDate AS Salman,(SELECT COUNT(*) FROM attendence_info WHERE AttStatus='P' AND AttDate=Salman) AS Monu,
    (SELECT COUNT(*) FROM attendence_info WHERE AttStatus='A' AND AttDate=Salman) AS Janu FROM attendence_info AS tab GROUP BY tab.AttDate"
);

Answer 1

以您形成查询的方式访问值选项卡。AttDAte 和 attData = Salman 彼此独立..就像是两个单独的查询..任何方式何时以及如果值匹配可能是您有 omogeneus行为。