Tag:Column value

  • Difference between count (*) and count (1) and count (column name)

    Time:2021-9-11

    Execution effect: Count () includes all columns, which is equivalent to the number of rows. When calculating the results,Column values that are null are not ignored Count (1) includes ignoring all columns and using 1 to represent the code line. In the statistical results,Column values that are null are not ignored   The count (column name) […]

  • MySQL partition table

    Time:2021-6-9

    MySQL partitioning is to decompose a table into several smaller tables. Logically, there is only one table or index, but physically, the table or index may consist of multiple physical partitions. Each partition is physically independent. Mysql database partition type: Range partition: row data is put into a partition based on column values belonging to […]

  • Explan of MySQL

    Time:2020-11-22

    Explain usage The meaning of explain result field id select_type table type possible_keys key key_len ref rows filtered Extra Explain using examples summary Blog reference Extended reading Using explain to query and analyze SQL execution records, you can optimize the performance of SQL! Explain usage mysql> explain select * from students; +—-+————-+———-+——+—————+——+———+——+——+——-+ | id | […]

  • Listag of Oracle function

    Time:2020-10-19

    It can realize the splicing of column values. Let’s take a look at its specific usage. The official document explains its role as follows: For a specified measure, LISTAGG orders data within each group specified in the ORDER BY clause and then concatenates the values of the measure column. That is, within each group, listaggSort the column plants according to […]

  • Differences between count (1), count (*) and count (column name) in SQL

    Time:2020-10-1

    1. count(1) and count(*) When the amount of data in the table is larger, it takes more time to use count (1) than count (*) after analyzing the table! From the execution plan, count (1) and count (*) have the same effect. However, after analyzing the table, count (1) will take less time than count […]

  • Execution difference between count (1), count (*) and count (column name)

    Time:2020-1-1

    By bigosprite blog.csdn.net/iFuMI/article/details/77920767 Execution effect: 1. count(1) and count(*) When the amount of data in a table is larger, after analyzing the table, count (1) is used more than count()More time! In terms of execution plan, count (1) and count()The effect is the same. But after the table has been analyzed, count (1) will take […]