C language and SQL Server database technology

Data query Foundation

One 、 Query basis

1. The purpose of storing data in a database is to query

2. Query is the most attractive aspect of database

3. The essence of query is data filtering

4.select： choice , lookup

5.select Statement is used to query , The grammar is as follows

select  Name  from  Table name  [where  Conditions ] [order by  Sort columns  [asc / desc] ]

6. Query all rows and all columns

select * from Student

7. Query partial Columns

select stuName,stuAge from Student

8. Alias the list

select stuName as  full name  stuAge as  Age  from Student
select stuName name ,stuAge age from Student     --as  It can be omitted 
select stuName 'name' from Student      -- You can use single quotation marks , Prevent special symbols or spaces 

9. adopt where Some lines of condition query

select * from student where stuSex=' male '

10. Multiconditional query

select * from student where stuSex=' male ' and stuAge > 22

Two 、 Query basis

 1. Query null value

select * from BookInfo where bookAuthor is null

2. Use constant columns in queries

select stuName,stuAge,' Central Primary School ' as schoolName from Student

3. Columns can be calculated or connected , Generate a new column

select bookPrice * bookCount as totalMoney from BookInfo
select bookDesc=bookName + '-' + bookAuthor from BookInfo

4. Return the limited number of rows             

a、select top 5 * from Student        -- Return to the former 5 strip 
b、select top 20 perent * from Student     -- Return to the top percent 20 The data of  

 5. Sort

                   a、asc Expressing ascending order ,desc Representation of descending order

                   b、 The default is asc Ascending

                   c、 Use order  by To specify the rank

                   d、 Between multiple rows , Comma separated

3、 ... and 、 Use functions in queries 1

1. Output statement

                  a、select

                  b、print

2. String function

                  charIndex（"cd" , "abcdecdfg" , 4）      --》   Return the position of the found string , From the beginning

                  len（"abcd"）                                        --》    Find the string length

                  upper（）/ lower（）                            --》    Turn capitalization 、 Turn lowercase

                  ltrim（）/ rtrim（）                                --》    Remove the left side 、 Space on the right

                  right（"abcdef",3）                             --》      Intercept a string of specified length from right to left

                  replace（"abcdef","ab","xx"）          --》      Replace string

                  stuff（"abcdef ",2,3,"xxx"）           --》      Delete the specified length string from the specified position , Reinsert content

3. Mathematical functions

                  rand（）                                               --》    Generate 0-1 Between random float value

                  abs（-43）                                           --》    Find the absolute value

                  ceiling（43.5）                                     --》    Rounding up

                  floor（43.5）                                        --》    Rounding down

                  power（5,2）                                     --》    Exponentiation

                  round（43.543,2）                            --》    Rounds a numeric expression to the specified precision

                  sign（-1）                                            --》    For integers, return +1, A negative number returns -1,0 return 0,                                                                                          Judge whether the number is positive or negative

                  sqrt（9）                                              --》    take a square root

Four 、 Use functions in queries 2

1. Date function

                  getDate（）                                          --》    Get system current time

                  dateAdd（mm,4,"1990-05-06"）     --》    Add the specified value to the specified part of the date

                  dateDiff（mm,"1990-05-01","1991-05-01"）       --》    Find the difference between two dates

                  dateName（dw,"1990-01-02"）         --》    The string form of the specified date part in the date

                  datePart（year,"1990-01-02"）          --》    The integer form of the specified date part of a date

2. Some parameters of date and their abbreviations

            year(yy,yyyy)                       weckday(dw,w)                        hour(hh)

            quart(qq,q)                          month(mm,m)                           minute(mi,n)

            dayofyear(dy,y)                   second(ss,s)                             day(dd,d)

            millisecond(ms)                     week(wk,ww)                             day(dd,d)

3. System function

                  convert（varchar(10),12345）            --》    Data type conversion

                  current_user                                           --》    Returns the current user name

                  dataLength（" I s"）                              --》    Returns the number of bytes occupied by the expression

                  host_name（）                                      --》    Returns the current computer name

                  system_user                                          --》    Return the current login