MySQL-DQL-aggregate function

Table of contents

aggregate function

Precautions

---

• aggregate function

  • Introduction: Take the data in the table as a whole and perform longitudinal calculations
  • Syntax: select aggregate function (field list) from table name
  • SUM summation
  • COUNT statistics
    • specific code

    • -- 1.统计该企业员工数量--count
      -- count(字段)
      select count(id)
      from tb_emp;
      -- count(常量)
      select count('A')
      from tb_emp;

  • AVG average
    • specific code

    • -- 4.统计该企业ID的平均值--avg
      select avg(id)
      from tb_emp;

  • MAX maximum value
    • specific code

    • -- 3.统计该企业最迟入职员工--max
      select max(entrydate)
      from tb_emp;

  • MIN minimum value
    • specific code

    • -- 2.统计该企业最早入职员工--min
      select min(entrydate)
      from tb_emp;

• Precautions

  • Null values ​​do not participate in the operation of all aggregate functions
  • Statistics can be used: count (*), count (field), count (constant), it is recommended to use count (*)