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子查询及LEFT JOIN的组合使用 - CSDN博客
Web一般情况下,一个join连接会生成一个MapReduce job任务,如果join连接超过2张表时,Hive会从左到右的顺序对表进行关联操作,上面的SQL,先启动一个MapReduce job任务对表employee和dept进行连接操作,然后在启动第二个MapReduce job对第一个MapReduce job输出的结果和表salary进行连接操作。 WebJun 5, 2024 · Hive converts joins over multiple tables into a single map/reduce job if for every table the same column is used in the join clauses e.g. SELECT a.val, b.val, c.val FROM a JOIN b ON (a.key = b.key1) JOIN c ON (c.key = b.key1) is converted into a single map/reduce job as only key1 column for b is involved in the join. On the other hand. rajiv bhuva npi
会引起全表扫描的几种SQL 以及sql优化 (转) - myseries - 博客园
WebAug 6, 2024 · Hive仅在FROM子句中支持子查询(从Hive 0.12版本开始)。必须为子查询指定名称,因为FROM子句中的每个表都必须具有名称。子查询 SELECT 列表中的列必须 … WebDec 14, 2024 · 一、join与left join的全称. JOIN是INNER JOIN的简写,LEFT JOIN是LEFT OUTER JOIN的简写。 二、join与left join的应用场景. JOIN一般用于A表和B表都存在的 … WebMar 31, 2024 · This is easy - left outer join! select * from A left join B on A.idA = B.idB However, what if I need to get v1 = v2 ? I thought that I could just use where. select * from A left join B on A.idA = B.idB where B.id is null or A.v1 = B.v2 Unfortunately, this removes all rows from the left table (A) that did not match any on B (in this example, idA ... dream dramalist