文章目录 示例 解释 一条select语句在MySQL中的奇幻之旅 示例 explain select * from emp; 解释 列(Column) 含义(Meaning) id The SELECT...type:此列是在优化SQL语句时最需要关注的列之一,此列显示了查询使用了何种类型。...index_merge:查询语句使用了俩个以上的索引,常见在使用and、or会出现,官
range:索引范围查询,常见于使用=,<>,>,>=,<,<=,ISNULL,<=>,BETWEEN,IN()或者like等运算符的查询中。 index:索引全表扫描,把索引从头到尾扫一遍 all:全表扫描,性能最差。 key:上面写着 rows:这是mysql估算的需要扫描的行数(不是精确值)。这个值非常直观显示 SQL 的效率好坏, 原则上 rows 越少越好。
See below for the meaning. For CROSS JOIN, none of these clauses may appear. A JOIN clause combines two FROM items, which for convenience we will refer to as "tables", though in reality they can be any type of FROM item. Use parentheses if necessary to determine the order of nesting....
lock_owner_type data NULL c--m Cursor 2 sqlserver lock_deadlock event o--c resource_type lock_resource_type data NULL c--m PAGE 6 Therefore, on your CREATE EVENT SESSION statement, in its ADD EVENT WHERE clause, you could put: WHERE( ... resource_type ...
OR there is an “empty” record for authors without books-- (“empty” meaning that all book columns are NULL) author LEFT OUTER JOIN book ON author.id = book.author_id SEMI JOIN 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...
For theINNERandOUTERjoin types, a join condition must be specified, namely exactly one ofNATURAL,ONjoin_condition, orUSING (join_column[, ...]). See below for the meaning. ForCROSS JOIN, none of these clauses can appear. AJOINclause combines twoFROMitems, which for convenience we will ...
To average an array we just average each value independently, but managing nested types in SQL is notoriously a PITA. I searched around and didn’t find any good answers on StackOverflow or the net that I liked, so I thought I would take a crack at a “pure” SQL approach. ...
1、 FROM 才是 SQL 语句执行的第一步,并非 SELECT 。数据库在执行 SQL 语句的第一步是将数据从硬盘加载到数据缓冲区中,以便对这些数据进行操作。(译者注:原文为“The first thing that happens is loading data from the disk into memory, in order to operate on such data.”,但是并非如此,以 Oracle 等...
Transact-SQL Index .NET Framework error occurred during execution of user-defined routine or aggregate "geography" 'IF EXISTS(SELECT COUNT(1))' VS 'IF EXISTS(SELECT 1) ' 'Incorrect syntax near' error while executing dynamic sql 'INSERT EXEC' within a function did not work 'Sort' in exuc...
-- In the results returned by the GROUP BY ALL example, notice that -- TotalDueByOrderDate was NULL for those order dates not included -- in the WHERE clause. This does not mean they have zero rows, but -- instead, that data is not displayed for them. ...