SQL server functions

Writing time ：2022 year 6 month 8 Japan

                    SQL Server  function 

SQL Contains the following types of functions ：
1、 Aggregate functions ： Returns the summary value .
2、 Type conversion function ： Convert one data type to another .
3、 Date function ： Processing date and time .
4、 Mathematical functions ： Perform arithmetic operations .
5、 String function ： The string 、 Binary data or expressions perform operations .
6、 System function ： Return from database at SQL Server The value in 、 Special information about objects or settings .
One 、 Aggregate functions ：
It returns a value for each rowset it applies .AVG( expression ) Returns the average of all values in the expression . Only for numeric columns and automatically ignore NULL value . Returns the average price for each product category ：
SELECT
category_name,
CAST(ROUND(AVG(list_price),2) AS DEC(10,2))
avg_product_price
FROM production.products p
INNER JOIN production.categories c
ON c.category_id = p.category_id
GROUP BY
category_name
ORDER BY
category_name;

COUNT( expression ) Return non in expression NULL The amount of value . Can be used for numeric and character columns . Count the number of customer calls

select COUNT(phone) from sales.customers

COUNT(*) Returns the number of rows in the table ( It includes NULL Columns of values ). Return the number of each brand and product . Besides , It only returns a product quantity greater than 20 The brand of ：

SELECT
brand_name,
COUNT() product_count
FROM production.products p
INNER JOIN production.brands c
ON c.brand_id = p.brand_id
GROUP BY brand_name
HAVING
COUNT() > 20
ORDER BY
product_count DESC;

MAX( expression ) Returns the maximum value in the expression , Ignore NULL value . Can be used for digital 、 Character and date time Columns . The following statement gets the brand name and maximum price of each brand ：
SELECT
brand_name,
MAX(list_price) max_list_price
FROM production.products p
INNER JOIN production.brands b
ON b.brand_id = p.brand_id
GROUP BY
brand_name
ORDER BY
brand_name;
Two 、 Type conversion function ：
Put the string ’100.125456’ Convert to Numeric type , Retain 2 Decimal place
select CONVERT(numeric(10,2) ,‘100.125456’);
Put the string ’100.125456’ Convert to Numeric type , Retain 3 Decimal place
select cast(‘100.125456’ as numeric(10,3))
String to time , Year of return
select year(cast(‘2022-03-13 12:00:30’ as date))
3、 ... and 、 Date function :
Because the date cannot directly execute the arithmetic function , So the date function is very useful .GETDATE() Current system date .
select GETDATE() -- result ：2019-05-07 18:34:27.343
DATEADD( Date part ,number,date) Returns a number with a specified number (number) Date (date), This number is added to the specified date section (datepart)
select DATEADD(dd, 5, getdate()) -- increase 5 Days time
DATEDIFF( Date part ,date1,date2) Returns the difference between the specified date parts of two dates .
select DATEDIFF(mm, ‘2010-1-1’, ‘2010-3-1 00:00:00’) -- result ：2
DATENAME( Date part ,date) Returns the string form of the date part of the date .
select DATENAME(dw,GETDATE()) -- result ： Tuesday
notes ：DATENAME and DATEPART The difference between , The returned value types are different , One is VARCHAR One is INT, In addition, the week will be expressed in local language
Four 、 Number function ：
Perform algebraic operations on numeric values .
ABS(num_expr) Returns the absolute value of a numeric expression .
select ABs(-1.2) -- Return the absolute value 1.2
FLOOR(num_expr) Returns the largest integer less than or equal to a numeric expression
select floor(1.2) -- return 1
CEILING(num_expr) Returns the smallest integer greater than or equal to a numeric expression
select CEILING(1.2) -- return 2
RAND([seed]) Random return to 0-1 Approximate floating point values between , It can be done to seed Specify as an integer expression ( Optional ).
select round(rand()*40,0)+60 -- Random generation 60-100 The integer of
ROUND(num_expr,length) Intercepts the specified integer length for a numeric expression , Returns the rounded value .
select ROUND(list_price,1) from production.products

5、 ... and 、 String function :
Can be used for binary and varbinary Data type column , But mainly for char and varchar data type .Expr1+expr2 Returns a string in the combined form of two expressions . Be careful int type
select ‘b’+‘a’;
select ‘b’+1;
LEN(char_expr) Returns the length of a character expression .
select len(‘123456’)
LOWER(char_expr) Convert all character expressions to lowercase .
select LOWER(‘ABc’)
LTRIM(char_expr) Returns a character expression that removes the preceding space .
select LTRIM(’ abc d e’)
6、 ... and 、 System function ：
Used to return metadata or configuration settings .COALESCE(expr1,expr2, xprN) Return the first non NULL expression .
select coalesce(null,1,1+1)
select coalesce(2,null,1+2)
select coalesce(null,null,1+2)
DATALENGTH(‘expr’) Returns the actual length of any type of data .
select DATALENGTH(‘ in ’); --2 byte
select DATALENGTH(‘A’); – 1 byte
ISNULL(expr,value) Replace with the specified value NULL expression .
select isnull(phone,‘unknow’) from sales.customers;
NULLIF(expr1,expr2) Expr1 And Expr2 When equal , return Null, Otherwise, the first expression is returned .
select nullIf(len(first_name),len(last_name)) from sales.customers
row_number Pagination row_number Has a very wide range of uses , It can be used to achieve web Paging of programs , It will generate a sequence number for each row of records queried , Sort in order and don't repeat , Pay attention to row_number Function must use over Clause to select a column , Or several columns can be sorted to generate sequence numbers
SELECT * FROM(
SELECT row_number() OVER(
ORDER BY list_price )
AS rowNum, product_id, product_name, list_price
FROMproduction.products ) a
WHERE rowNum > (3 -1) * 10
AND rowNum <= 3 * 10