Sqlserver common statements and functions

One 、SqlServer Common sentences

1、 Delete all tables

1.1、 Delete foreign key constraints for all tables （ Prevent table deletion failure due to foreign key constraints ）

DECLARE c1 cursor for
select 'alter table ['+ object_name(parent_obj) + '] drop constraint ['+name+']; '
from sysobjects
where xtype = 'F'
open c1
declare @c1 varchar(8000)
fetch next from c1 into @c1
while(@@fetch_status=0)
begin
exec(@c1)
fetch next from c1 into @c1
end
close c1
deallocate c1

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1.2、 Delete all stored procedures

use  Database name ( Is the name of the database where the table is to be deleted )
GO
declare @sql varchar(8000)
while (select count(*) from sysobjects where type='P')>0
begin
SELECT @sql='drop Procedure ' + name
FROM sysobjects
WHERE (type = 'P')
ORDER BY 'drop Procedure ' + name
exec(@sql)
end

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1.3、 Delete all tables

use  Database name ( Is the name of the database where the table is to be deleted )
GO
declare @sql varchar(8000)
while (select count(*) from sysobjects where type='U')>0
begin
SELECT @sql='drop table ' + name
FROM sysobjects
WHERE (type = 'U')
ORDER BY 'drop table ' + name
exec(@sql)
end

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Two 、SqlServer Common functions

1、concert() function

concert() Function is a general function that converts dates to new data types .concert() Functions can display dates in different formats / Time data .

1.1、 grammar

CONVERT(data_type(length),data_to_be_converted,style)

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data_type(length) Specify the target data type （ With optional length ）.data_to_be_converted Contains values that need to be converted .style Specified date / Time output format .

serviceable style value ：

Style ID	Style Format
100 perhaps 0	mon dd yyyy hh:miAM （ perhaps PM）
101	mm/dd/yy
102	yy.mm.dd
103	dd/mm/yy
104	dd.mm.yy
105	dd-mm-yy
106	dd mon yy
107	Mon dd, yy
108	hh:mm:ss
109 perhaps 9	mon dd yyyy hh:mi:ss:mmmAM（ perhaps PM）
110	mm-dd-yy
111	yy/mm/dd
112	yymmdd
113 perhaps 13	dd mon yyyy hh:mm:ss:mmm(24h)
114	hh:mi:ss:mmm(24h)
120 perhaps 20	yyyy-mm-dd hh:mi:ss(24h)
121 perhaps 21	yyyy-mm-dd hh:mi:ss.mmm(24h)
126	yyyy-mm-ddThh:mm:ss.mmm（ There are no spaces ）
130	dd mon yyyy hh:mi:ss:mmmAM
131	dd/mm/yy hh:mi:ss:mmmAM

1.2、 example

The following script uses concert() Function to display different formats . We will use GETDATE() Function to get the current date / Time ：

CONVERT(VARCHAR(19),GETDATE())
CONVERT(VARCHAR(10),GETDATE(),110) 
CONVERT(VARCHAR(11),GETDATE(),106)
CONVERT(VARCHAR(24),GETDATE(),113)

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The results are similar ：

Dec 29 2008 11:45 PM
12-29-2008
29 Dec 08
29 Dec 2008 16:25:46.635

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2、count() function

2.1、count(1) and count(*)

When the table has more data , After analyzing the table , Use count(1) It's better than using count( * ) It takes a lot of time ！ In terms of execution plan ,count(1) and count( * ) The effect is the same . But after the table has been analyzed ,count(1) than count( * ) We need less time （1w The amount of data ）, But not much . If count(1) It's a clustered index ,id, That must be count(1) fast . But the difference is very small . because count( * ), Automatically optimize the assigned field . So there's no need to count(1), use count( * ),sql It will help you optimize therefore ：count(1) and count( * ) Basically no difference ！

2.2、count(1) and count( Field )

The main difference between the two is （1） count(1) Will count all the records in the table , The containing field is null The record of . （2） count( Field ) Count the number of times the field appears in the table , The ignored field is null The situation of . That is to say, do not count the fields as null The record of .

2.3、count(*) and count(1) and count( Name ) difference

On the effect of execution ： count(*) It includes all the columns , Equivalent to the number of lines , When it comes to statistical results , Column values of... Are not ignored NULL count(1) Including ignoring all columns , use 1 Represents the line of code , When it comes to statistical results , Column values of... Are not ignored NULL count( Name ) Only include the column of the list , When it comes to statistical results , Will ignore null column values （ The null here is not just an empty string or 0, It means null） Count of , That is, the value of a field is NULL when , No statistics .

In terms of execution efficiency ： The column name is primary key ,count( Name ) than count(1) fast Column name is not primary key ,count(1) than count( Name ) fast If the table has multiple columns and no primary key , be count（1） Is superior to the execution efficiency count（ * ） If there is a primary key , be select count（ Primary key ） The efficiency of execution is optimal If the table has only one field , be select count（*） The optimal .

3、len() function

len() Function returns the length of the value in the text field .

4、cast() function

4.1、CAST() The argument to the function is an expression , It includes the use of AS Key separated source and target data types . The following example is used to put a text string ’12’ Convert to integer :

SELECT CAST('12' AS int)

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4.2、 The return value is an integer value 12. If you try to convert a string representing a decimal to an integer value , What will happen ？

SELECT CAST('12.5' AS int)  

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4.3、CAST() Functions and CONVERT() Functions cannot perform rounding or truncation . because 12.5 Out-of-service int Data type , So a call to this function will cause an error :

Server: Msg 245, Level 16, State 1, Line 1 Syntax error converting the varchar value     '12.5' to a column of data type int.

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4.4、 To return a valid value , You must use a data type that can handle this value . For this example , There are multiple data types available . If you pass CAST() Function converts this value to decimal type , You need to first define decimal The precision of the value and the number of decimal places . In this case , The precision and decimal places are respectively 9 And 2. Precision is the total number of digits , Include the sum of the digits to the left and right of the decimal point . And the number of decimal places is the number to the right of the decimal point . This means that the largest integer value that this example can support is 9999999, And the smallest decimal is 0.01.

SELECT CAST('12.5' AS decimal(9,2))

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decimal The data type displays significant decimal places in the results grid : 12.50

4.5、 The default values for precision and scale are 18 And 0. If in decimal These two values are not provided in the type ,SQL Server The decimal part of the number will be truncated , Without making mistakes .

SELECT CAST('12.5' AS decimal)

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The result is an integer value ：12