SQL Server data type conversion function cast () and convert () explanation

In SQL Server, data type conversion functions, commonly used functions are cast() and convert().

• Comparison of cast() and convert() functions:

(1)cast is generally easier to use, and the advantage of convert is that it can format dates and values;

(2) convert is generally used to convert between date and time types and decimals, while cast is generally used to convert decimal values ​​and characters;

(3) convert is just a display conversion, and cast is a forced conversion.

One, cast() function

The cast() function is very simple to use, the syntax is:

cast(field name as conversion type)

Example:

select cast(123.45?as int)? ? ? ?--123, convert float type to int typeselect cast('123.447654' as int)????????-- Msg 245, Level 16, State 1, Line 1, Failed converting varchar value '2017.3' to data type int./*123.45 is of type float, and '123.447654' is of type char,Strings with decimal places cannot be directly converted to int type, but can be directly converted to decimal type.*/select cast('123.447654' as decimal)????????--123, decimal means that only the integer part is retained, and the fractional part is not retained.select cast('123.447654' as decimal(5,2))? ? ? ?/*123.45, convert the char type to a numeric type, 5 is the precision, which means the longest is 5 digits except the decimal point;2 is the number of decimal places, which means that there are two decimal places after the decimal point.And if you write select cast('123.447654' as decimal(5,3)),The error "Arithmetic overflow occurred while converting varchar to data type numeric" will be reported.Because during type conversion, the integer part will be satisfied first, and the integer part in '123.447654' already has 3 bits,The decimal part can only have 2 digits at most, and 3 decimal places cannot be reserved, all of which will report an error.At this time, it is necessary to improve the accuracy, that is, increase the value of 5.*/select cast('20210922' as date)? ? ? ?--2021-09-22, convert char type to date typeselect cast(getdate() as time)? ? ? ?--22:07:08.4200000, get the current time information without keeping the date

Second, convert() function

Syntax:
convert(data_type(length),expression,style)

value

Description

data_type(length)

Specifies the target data type (with an optional length).

expression

Specifies the value to be converted.

style

Specifies the output format of the date/time.

The following table shows the style values ​​of datetime or smalldatetime converted to character data:

Value
(century yy)

Value
(century yyyy)

input/output

Standard

-

0 or 100

mon dd yyyy hh:miAM (or PM)

Default

1

101

mm/dd/yy

USA

2

102

yy.mm.dd

ANSI

3

103

dd/mm/yy

British/French

4

104

dd.mm.yy

German

5

105

dd-mm-yy

Italian

6

106

dd mon yy

7

107

Mon dd, yy

8

108

hh:mm:ss

-

9 or 109

mon dd yyyy hh:mi:ss:mmmAM (or PM)

Default+millisec

10

110

mm-dd-yy

USA

11

111

yy/mm/dd

Japan

12

112

yymmdd

ISO

-

13 or 113

dd mon yyyy hh:mi:ss:mmm (24h)

14

114

hh:mi:ss:mmm (24h)

-

20 or 120

yyyy-mm-dd hh:mi:ss (24h)

-

21 or 121

yyyy-mm-dd hh:mi:ss.mmm (24h)

-

126

yyyy-mm-ddThh:mi:ss.mmm (no spaces)

ISO8601

-

130

dd mon yyyy hh:mi:ss:mmmAM

Hijiri

-

131

dd/mm/yy hh:mi:ss:mmmAM

Hijiri

Example:

SELECT CONVERT(INT, 25.65) --25select CONVERT(decimal, '123.45') -- 123select CONVERT(decimal(9,2), '123.45') -- 123.45SELECT CONVERT(VARCHAR(25),GETDATE()) --09 22 2021 10:31PMSELECT CONVERT(VARCHAR(24),GETDATE(),120) --2021-09-22 22:32:43, most commonly used, keep date and time informationSELECT CONVERT(VARCHAR(10),GETDATE(),102) --2021.09.22, only keep date informationSELECT CONVERT(VARCHAR(10),GETDATE(),111) --2021/09/22, only keep date informationSELECT CONVERT(VARCHAR(10),GETDATE(),108) --22:34:06, only keep time information

Refer to: SQL Server CONVERT() Function|Rookie Tutorial

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