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
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...
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。
I had a table with many instances of a primary key and an array. I needed to return one row with an averaged array in SparkSQL. 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...
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 ...
vol_id: each volunteer’s identification number, expressed with theintdata type. This column will serve as the table’sprimary key, meaning that each value will function as a unique identifier for its respective row. Because every value in a primary key must be unique, this column will also...
Here, the command selects rows if combinations ofcountryandfirst_nameare unique. Meaning, the result will include each pair ofcountryandfirst_nameonly once. Example: Selecting the unique combined fields DISTINCT With COUNT We can use SQLDISTINCTwith theCOUNT()function to count the number of uniqu...
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....
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...
There are no intermediate commits at the SQL layer during the DDL operation. 在DDL操作期间,SQL层没有中间提交。 Where applicable:在适用情况下 The state of data dictionary, routine, event, and loadable function caches is consistent with the status of the DDL operation, meaning that caches are up...