如果等号两边的值、字符串或表达式都为字符串,则MySQL会按照字符串进行比较,其比较的 是每个字符串中字符的ANSI编码是否相等。 如果等号两边的值都是整数,则MySQL会按照整数来比较两个值的大小。 如果等号两边的值一个是整数,另一个是字符串,则MySQL会将字符串转化为数字进行比较。 如果等号两边的值、字符串或表达...
When EXPLAIN is used with FOR CONNECTION connection_id rather than an explainable statement, it displays the execution plan for the statement executing in the named connection. See Section 8.8.4, “Obtaining Execution Plan Information for a Named Connection”. 当EXPLAIN与FOR CONNECTION connection_id...
更多select语句级别的优化细节 见(https://dev.mysql.com/doc/refman/8.0/en/select-optimization.html) 为了让读者对join优化 有更深的了解,章节里的sql例子,留了一些思考和动手的问题。可能大家得到的答案会不同,但探索未知的过程,方式应该是一样的。 join优化(Join Optimizations) MySQL可以使用Join Optimizations...
a good way to determine the best query plan.For example,SELECTc1FROMt1WHEREc1=x;mightreturn1row when x=50and a million rows when x=30.In such acase,you might need to use index hints to pass along advice about which lookup method is more efficientfora particular query.Cardinality can also...
set_child() : opt_explain_json_namespace::context, opt_explain_json_namespace::subquery_ctx set_child_def_version() : Mrg_child_def set_child_schema_name() : dd::Foreign_key_parent set_child_table_name() : dd::Foreign_key_parent set_chunk() : Clone_Task_Manager set_class_origins(...
创建方式2使用AS subquery 选项,将创建表和插入数据结合起来指定的列和子查询中的列要一一对应通过列名和默认值定义列CREATE TABLE emp1 AS SELECT * FROM employees;CREATE TABLE emp2 AS SELECT * FROM employees WHERE 1=2; -- 创建的emp2是空表CREATE TABLE dept80...
5. MySQL subquery with EXISTS & NOT EXISTS The EXISTS and NOT EXISTS operators in SQL subquery return a Boolean value as a result, which can be either TRUE or FALSE. The following is the syntax to use EXISTS in a subquery: SELECT * (column1, …) FROM Tablename WHERE EXISTS (subquery...
MySQL can use this join type when the query uses only columns that are part of a single index. 触发条件:1)查询的字段是索引的一部分,覆盖索引。2)使用主键进行排序all A full table scan is done for each combination of rows from the previous tables. This is normally not good if the table ...
消息:撤销数据库时出错(can't rmdir '%s', errno: %d) 错误:1011 SQLSTATE: HY000 (ER_CANT_DELETE_FILE) 消息:删除'%s'时出错 (errno: %d) 错误:1012 SQLSTATE: HY000 (ER_CANT_FIND_SYSTEM_REC) 消息:无法读取系统表中的记录。 错误:1013 SQLSTATE: HY000 (ER_CANT_GET_STAT) ...
Subquery is not grouped (explicitly or implicitly) 3x: outer aggregated expression are not accepted 4. Subquery does not use HAVING 5. Subquery does not use windowing functions 6. Subquery predicate is (a) in an ON/WHERE clause, and (b) at the AND-top-level of that clause. Note for ...