Basic Queries — Built-in Functions

overview

1. Function overview

• A program or code that can be directly called by a program at the other end.
• Encapsulate a set of functional logic in the function body and expose the function name to the outside.

2. Advantages and disadvantages of functions

• advantage:

  • Code implementation details are hidden.
  • Improved code reusability and simplicity.

• Disadvantages: Poor portability, functions with similar functions may have different function names in different DBMSs

3. Classification of built-in functions

• One-line functions: string functions, numeric functions, date functions, message functions, control functions...
• Multi-row functions: aggregate functions.

4. Basic Grammar

select 函数名(实参列表) [from 表名];

Common one-line functions

1. String functions

• Length function: length,char_length

  • lengthGet the number of bytes, char_lengthget the number of characters.

  • select length('张三'); # 6
    select char_lebgth('张三'); # 2
    

• Splicing function:concat

  • Used to splice strings, you can specify what splicing

  • # 直接连接
    select concat(first_name, last_name) as fillname
    from employees;
    
    # 以'-'连接
    select concat(first_name, '-', last_name) as fillname
    from employees;
    

• Case conversion: upper,lower

  • upperConvert all to uppercase and lowerall to lowercase.

  • select upper("Hello World"); # HELLO WORLD
    select lower("Hello World"); # hello world
    

• Operand substring: substr,instr

  • substr: Take a substring, the index starts from 1

    # 截取从指定索引处后面所有字符
    select substr('good good study', 7); # ood study
    # 截取从指定索引处指定字符长度的字符
    select substr('day day up', 2, 5); # ay da
    

  • instr: Get the position where the substring appears and return the index of the first occurrence of the substring, if not found, return 0

    select instr('good good study', "good"); # 1
    select instr('good good study', "day"); # 0
    

• Remove extra spaces and pad to the specified length: trim, lpad,rpad

  • trim: Remove extra spaces, or certain characters (first or last).

    select trim("siu     "); # siu
    select trim('~' from "~~~siu~~~~~ "); # siu~~~~~ 
    

  • lpadleft padding, rpadright padding

    select lpad('batman', 10, '*'); # ****batman
    select rpad('batman', 10, '*'); # batman****
    

• replace:replace

  • replace some strings with others

  • select 'hello world' as "before", # hello world
    replace('hello world', 'world', 'mysql') as "after"; # hello mysql
    

2. Mathematical functions

• Rounding and Rounding: round, ceil,floor

  • roundrounding

    select round(2.5); # 3
    select round(2.4);# 2
    

  • ceilround up, floorround down

    select ceil(2.6); # 3
    select floor(2.6); # 2
    

• Truncated:truncate

  • Achieve the effect of retaining the required number of decimal places

  • select truncate(3.1415926, 2); # 3.14
    select truncate(3.1415926, 5); # 3.14159
    

• Absolute value and random number: abs(),rand()

  • abstake the absolute value

    select abs(-1.414); # 1.414
    

  • randtake random number

    select rand(); # 任意随机数
    select round(rand() * 10); # 1~10的随机数
    

3. Date function

• Get the current date and time

  • now: Returns the current system date and time.
  • curdate: Returns the current system date.
  • curtime: Returns the current system time.

  select now(); # 2023-06-06 09:36:54
  select curdate(); # 2023-06-06
  select curtime(); # 09:36:56
  

• Fetch partial information of time and date

  • Dates: year(), month(), day(), monthname()
  • Time: hour(), minute(), second()
  • Several special ones:
    • quarter(): Returns the quarter corresponding to the date.
    • weekofyear: Returns the week of the year.
    • dayofyear: Returns the day of the year.
    • dayofmonth: returns the day of the month
    • dayofweak: returns the day of the week (Sunday is 1)

• Conversion between time and seconds

  • The number of seconds is only related to the time, not the date.

  • Conversion formula: h×3600 + m×60 + s.

  • time_to_sec Convert time to seconds and sec_to_timeconvert seconds to time.

    select curtime(), time_to_sec(now()), sec_to_time(time_to_sec(now()));
    # 09:46:04  35164  09:46:04    
    

• format

  • Display date and time in a custom format

  • placeholder in fromt format string

    %Y 年，四位数字
    %y 年，后两位数字
    %m 月 ，数字[1-12]
    %M 月名 
    %d 日，月份里的第几天，两位数字[1-31]
    %D 带有英文前缀的月中的天 
    %H 小时，24进制 [0-23]
    %h 小时，12进制[0-11]
    %i 分钟 [0-59]
    %s | %S 秒 0-59
    

  • date_format(date, format): formatted date

    select date_format(now(), '%m/%d-%y')
    from dual; # 06/06-23
    

  • time_format(date, format): formatted time

    select
      time_format(curtime(), '%h 点 %i 分 %s 秒')
    from dual; # 09 点 55 分 05 秒
    

• String to date:str_to_date

  select
    str_to_date('2000-1-2', '%Y-%m-%d'),
    str_to_date('May 1, 2023', '%M %d,%Y')
  from dual;
  # 2000-01-02  2023-05-01 
  

4. Information function

• versionQuery MySQL version number
• userQuery the currently logged in account
• databaseQuery the currently used database

5. Process control function

• iffunction

  • grammar:if(expr, trueValue, falseValue)

  • select if(10 < 5, 'true', 'false') as "10 < 5"; #false
    

• casefunction

  • grammar

    case [field_x]
      when XX then [statement-1]
      when XX then [statement-2]
      ...
      else [statement-n]
    end
    

  • Required when XX is a specific value case [field_x], required when XX is a conditional expressioncase

    /*
     * 查询员工的工资、部分编号和 new_salary，其中 new_salary 的计算如下
     *   部门编号如果是 30，new_salary 为 1.2 倍工资
     *   部门编号如果是 40，new_salary 为 1.25 倍工资
     *   部门编号如果是 50，new_salary 为 1.3 倍工资
     *   其他部门，new_salary 为原工资
    */
    
    select salary, department_id,
      case department_id
        when 30 then salary*1.2
        when 40 then salary*1.25
        when 50 then salary*1.3
        else salary
      end as new_salary
    from employees;
    
    /*
     * 查询员工的名字、工资以及工资级别 grade，grade 计算如下
     *   如果工资大于 20000，grade 为 A 级
     *   如果工资大于 15000，grade 为 B 级
     *   如果工资大于 10000，grade 为 C 级
     *   否则，grade 为 D 级
    */
    
    select first_name, salary,
      case
      when salary > 20000 then 'A'
      when salary > 15000 then 'B'
      when salary > 10000 then 'C'
      else 'D'
      end as grade
    from employees;
    

Common multi-line functions

1.Overview

• Multi-row functions are also known as aggregate functions.
• Used for statistics.
• Classification: sumsummation, avgaverage value, maxmaximum value, minminimum value, countnumber of calculations.

2. Features

• sum and avg are generally used to process numeric values.
• max, min, count can handle any type.
• The above grouping functions all ignore null values.
• It can be used with distinct to implement deduplication operations.
• The fields queried together with the grouping function are required to be the fields after group by.

3. Examples

• easy to use

  select
    sum(salary),
    avg(salary),
    min(salary),
    max(salary),
    count(salary)
  from employees;
  

• with distinctcollocation deduplication

  select
    sum(distinct salary),
    avg(distinct salary),
    min(distinct salary),
    max(distinct salary),
    count(distinct salary)
  from employees;
  

• About countfunctions

  • count(expression): Returns NULLthe number of rows that do not contain a value.
  • count(*): The number of rows in the returned result set, NULLthe number of rows that contain values, that is, all rows.
  • count(distinct expression): Returns NULLthe number of unique rows that do not contain a value.

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