When working with SQL LEFT JOINS keep in mind your match from one table to another may match multiple rows. Meaning, your result may have more rows in the result that you have in either table. When columns don’t match, the query the left row, and NULL for the right table columns… ...
经和运维配合查看,发现是SQL语句问题,有个sql查询脚本执行竟然消耗了40秒,我拿出来自己执行发现亦是如此。 sql大致情况就是有个left join了一张表,有10几万数据,使用explain查看,就是这个表执行消耗了近40秒。 解决 由于是生产环境,涉及数据隐私,具体sql就不贴了,解决思路就是使用explain+SQL语句查看哪个执行是全...
-- This table reference contains authorsandtheir books-- Thereisone recordforeach bookandits author.-- ... OR thereisan"empty"recordforauthors without books-- ("empty"meaning that all book columns are NULL)author LEFT OUTER JOIN book ON author.id = book.author_id 这种连接关系在 SQL 中有...
However, just because they are functionally the same, in that they produce the same results, does not mean the two kinds of clauses have the same semantic meaning.回答2outer join的时候,比如left outer join, 会以左表为主Does not matter for inner joins 【做inner join的时候没影响,但是做outer...
Sure, here is the text with the same meaning: The output shows only nine employees, but there are actually 15 employees in the database. The missing employees are those with IDs 3, 4, 7, 9, 11, and 12. These employees are not included in the result because they have NULL values in...
-- ("empty" meaning that all book columns are NULL) author LEFT OUTER JOIN book ON author.id = book.author_id 这种连接关系在 SQL 中有两种表现方式:使用 IN,或者使用 EXISTS。“ SEMI ”在拉丁文中是“半”的意思。这种连接方式是只连接目标表的一部分。这是什么意思呢?再想一下上面关于作者和书名...
SQL reserved words have special meaning in SQL and should not be used for any other purpose. For example,SELECTandUPDATEare reserved words and should not be used as table names. A SQL statement must be the equivalent of a complete SQL sentence, such as: ...
sparksql 2 left join性能优化 sparksql性能调优 Spark_SQL性能调优 众所周知,正确的参数配置对提升Spark的使用效率具有极大助力,帮助相关数据开发、分析人员更高效地使用Spark进行离线批处理和SQL报表分析等作业。 性能调优选项 几种压缩选项的特点 spark.sql.parquet.compressed.codec 默认值为snappy 这个参数代表使用哪...
Okay, so here’s my copy of my query. Now I’m going to make it a left outer join. Bear in mind, just changing the word from right to left give the query an entirely different meaning. now what we are doing is looking for all product vendors that we have in our system that may...
LEFT (OUTER) JOIN: Returns all records from the left table, and the matched records from the right table RIGHT (OUTER) JOIN: Returns all records from the right table, and the matched records from the left table FULL (OUTER) JOIN: Returns all records when there is a match in either left...