Analysis of MySQL group by group aggregation and subdivision aggregation, mysqlgroup
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MySQL optimizes group by (loose index scan and compact index scan) and mysqlgroup
The most common method to satisfy the group by clause is to scan the entire table and create a new temporary table. All rows in each GROUP in the table should be
The most common way to satisfy a GROUP BY clause is to scan the entire table and create a new temporary table in which all rows of each group should be contiguous, and then use the temporary table to locate the group and apply the cumulative
The most general way to satisfy the GROUP BY clause is to scan the entire table and create a new temporary table, all rows of each group in the table should be contiguous, and then use the temporary table to locate the group and apply the cumulative
1. Using Loose (Loose) index scans to implement GROUP by
What is a loose index scan implementation Group by? In fact, when MySQL fully utilizes the index scan to implement group by, it does not need to scan all index keys that meet the criteria to
Comparison and usage of distinct and group by statements in MySQL, mysqldistinct
The data table records the bibliography used for user verification. Now I want to retrieve all the bibliography and use DISTINCT and group by to get the expected
1. Aggregation functions (Aggregate function)MySQL (5.7) the list of aggregate functions given in the official documentation (image) is as follows:Click https://dev.mysql.com/doc/refman/5.7/en/group-by-functions.html for details.Unless otherwise
You can use the aggregate function count (*) in Mysql to implement this function. For example, to query the records with the most frequently displayed names in the user table, you can group the records by group by name first, use count to calculate
Mysql database groups are divided into three methods. Temporary tables are used for group by, compact (Tight) index scanning, and group by and Loose index scanning, but I often use
1. Use Loose index scanning to implement GROUP
What is loose index
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