MySQL keyword

1.TIMESTAMPDIFF

• effect

​ * Calculate the integer difference between function dates or date expressions

TIMESTAMPDIFF(PARAM,DATETIME1,DATETIME2)

#DATETIME1, and DATETIME2 Time difference between , With param In units of time 
# The supported time units are ：
#FRAC_SECOND  Interval milliseconds ,*SECOND  second ,*MINUTE  minute ,*HOUR  Hours ,*DAY  God ,*WEEK  week ,*MONTH  month ,#*QUARTER  quarter ,*YEAR  year 

2.AVG

• effect

​ * averaging , You can find the average value of a certain field , You can also find the average of all the values of a function formula

AVG( Field )
AVG( Value of formula )

3.IF(trueOrfalse,param2,param3)

• effect

  * Make a judgment on the condition , If the judgment is true , If the expression is true The return value is param2, If the expression is false The return value is param3

IF( expression , The return value when the expression is true , The return value when the expression is false )

4.substring_index(str,delim,count)

• effect

​ * Cut string

substring_index(str,delim,count)
#str  Cutting objects   String object to intercept 
#delim  Separator   With what mark to cut the target object 
#num  Serial number   Used to control cutting objects , When positive ( barring 0) Cut from left to right , When it is negative, from right to left 

5.CAST( target AS CHAR)

• effect

  ​ * Convert the results of the query into char type

CAST( expression  AS CHAR)

6.CONCAT(char c1, char c2, …, char cn)

• effect

​ * String splicing

CONCAT(char c1, char c2, …, char cn)

7.DATE_SUB(date,INTERVAL expr type)

• effect

​ * Function to subtract a specified time interval from a date

DATE_SUB(date,INTERVAL expr type)
#date: Legal date expression 
#expr: Number 
#type: type --》MICROSECOND SECOND MINUTE HOUR DAY WEEK MONTH QUARTER YEAR SECOND_MICROSECOND MINUTE_MICROSECOND MINUTE_SECOND HOUR_MICROSECOND HOUR_SECOND HOUR_MINUTE DAY_MICROSECOND DAY_SECOND DAY_MINUTE DAY_HOUR YEAR_MONTH

8. obtain datatime Type year month day

• 1. Directly through year(),month(),day()

  select year(dattime Type field ),month(dattime Type field ),day(dattime Type field ) from ' Table name '
  

• 2. Use datepart Keyword get date

  datepart('year',' Field name '),datepart('month',' Field name '),datepart('day',' Field name ')
  

9.mysql Keep decimals in

1. Use round(x,d) -->x For the object to keep ,d Is the decimal to keep

Be careful : Keep decimals in this way , Don't add when the number of digits is not enough 0, The final retention result will be rounded ,d It could be negative , Take the decimal point as 0 The dividing line of , Below the corresponding digits on the left becomes 0

2.turncate(x,d) and round similar , It's just not rounded

3.**format(x,d)** Force to keep the corresponding decimal places. If the decimal is not enough, fill in zero , The return value is string type

4.**convert(num,type)** Equivalent to interception

CONVERT(100.3465,DECIMAL(10,2))