AI代码解释 DO$$DECLAREv_tsTIMESTAMP;v_repeatCONSTANTINT:=25;recRECORD;BEGIN--Repeat the whole benchmark several times to avoid warmup penaltyFORrIN1..5LOOPv_ts:=clock_timestamp();SETenable_memoize=OFF;FORiIN1..v_repeatLOOPFORrecIN(SELECTt.*FROMtJOINuONt.j=u.j)LOOPNULL;ENDLOOP;ENDLOOP...
理论上来说,嵌套循环连接等于两个嵌套的循环,比如说employees表和departments表之间的嵌套循环连接,可以看成是: FOR erow IN (select * from employees where X=Y) LOOP FOR drow IN (select * from departments where erow is matched) LOOP output values from erow and drow END LOOP END LOOP 可以看出,外...
<selectid="findById"resultMap="user"parameterType="integer"> select * from userwhereid=#{id}</select> 测试(可以成功查询到所有信息): String config ="sqlMapConfig.xml";InputStream inputStream = Resources.getResourceAsStream(config);sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream)...
多表之间的连接有三种方式:Nested Loops,Hash Join 和 Sort Merge Join.具体适用哪种类型的连接取决于 当前的优化器模式 (ALL_ROWS 和 RULE) 取决于表大小 取决于连接列是否有索引 取决于连接列是否排序 下面来介绍三种不同连接工作方式的不同: 实验sql 假如有10000个城市,对应于10个国家(此例子仅仅可以解释join...
DO $$DECLAREv_ts TIMESTAMP;v_repeat CONSTANT INT := 25;rec RECORD;BEGIN-- Repeat the whole benchmark several times to avoid warmup penaltyFOR r IN 1..5 LOOPv_ts := clock_timestamp();SET enable_memoize = OFF;FOR i IN 1..v_repeat LOOPFOR rec IN (SELECT t.*FROM t JOIN u ON...
上面的sql大致流程是: 将t2 的所有数据放入到登录后复制join_buffer中 将join_buffer 中的每一条数据,跟表t1中所有数据进行比较 返回满足join 条件的数据 3、Index Nested-Loop Join 算法 登录后复制-- a字段有索引EXPLAINselect*fromt1innerjoint2ont1.a= t2.a;-- 执行结果+---+---+---+---+---+...
我们都知道SQL的join关联表的使用方式,但是这次聊的是实现join的算法,join有三种算法,分别是Nested Loop Join,Hash join,Sort Merge Join。 MySQL官方文档中提到,MySQL只支持Nested L… 菜五岁 不要再问我 in,exists 走不走索引了... 里奥ii发表于Java学... Select for update使用详解 前言近期开发与钱相关的...
We can define the syntax of a nested CASE WHEN statement as shown in the following: SELECT CASE WHEN condition1 THEN result1 WHEN condition2 THEN result2 ELSE result3 END FROM table_name; Let’s take a real-world database such as the Sakila database to learn how to accomplish this: ...
SELECT TO_CHAR(MAX(SUM(salary))) Max_Salary FROM teams t JOIN playersalary p ON t.id = p.team_id GROUP BY name; Output: 在这里我可以拿到最高工资,但我不能显示球队的名字。 #2: SELECT name, salary FROM teams t JOIN playersalary p ON t.id = p.team_id ...
Subqueries are nested in the WHERE clause, and the subquery result is used as the filtering condition. Syntax SELECT[ALL|DISTINCT]attr_expr_listFROMtable_referenceWHERE{col_nameoperator(sub_query)|[NOT]EXISTSsub_query}; Keyword All is used to return repeated rows. By default, all repeated rows...