Practical SQL statement Daquan _ MySQL

Practical SQL statement Daquan bitsCN.com

I. Basics

1. description: create a database

Create database database-name

2. description: Delete a database.

Drop database dbname

3. description: back up SQL server

--- Create a device for the backup data

USE master

EXEC sp_addumpdevice 'disk', 'testback', 'C:/mssql7backup/MyNwind_1.dat'

--- Start backup

Backup database pubs TO testBack

4. description: create a new table.

Create table tabname (col1 type1 [not null] [primary key], col2 type2 [not null],...)

Create a new table based on an existing table:

A: create table tab_new like tab_old (use the old table to create A new table)

B: create table tab_new as select col1, col2... From tab_old definition only

5. description: delete a new table.

Drop table tabname

6. description: add a column.

Alter table tabname add column col type

Note: columns cannot be deleted after they are added. After columns are added to DB2, the data type cannot be changed. the only change is to increase the length of the varchar type.

7. description: add a primary key: Alter table tabname add primary key (col)

Delete a primary key: Alter table tabname drop primary key (col)

8. description: create an index: create [unique] index idxname on tabname (col ....)

Delete index: drop index idxname

Note: The index cannot be changed. to change the index, you must delete it and recreate it.

9. description: create view viewname as select statement

Delete view: drop view viewname

10. description: several simple basic SQL statements

Select: select * from table1 where range

Insert: insert into table1 (field1, field2) values (value1, value2)

Delete: delete from table1 where range

Update: update table1 set field1 = value1 where range

Search: select * from table1 where field1 like '% value1 %' --- the like syntax is very subtle, query information!

Sort: select * from table1 order by field1, field2 [desc]

Total: select count as totalcount from table1

Sum: select sum (field1) as sumvalue from table1

Average: select avg (field1) as avgvalue from table1

Max: select max (field1) as maxvalue from table1

Min: select min (field1) as minvalue from table1

11. description: several advanced query operators

A: UNION operator

The UNION operator combines two other result tables (such as TABLE1 and TABLE2) and removes any duplicate rows from the table to generate a result table. When ALL is used together with UNION (that is, union all), duplicate rows are not eliminated. In either case, each row of the derived table is from either TABLE1 or Table2.

B: random T operator

The distinct T operator derives a result table by including all rows in Table 1 but not in table 2 and eliminating all repeated rows. When ALL is used with distinct T (distinct t ALL), duplicate rows are not eliminated.

C: INTERSECT operator

The INTERSECT operator derives a result table by only including the rows in TABLE1 and TABLE2 and eliminating all repeated rows. When ALL is used with INTERSECT (intersect all), duplicate rows are not eliminated.

Note: the query results of several computation words must be consistent.

12. Note: Use external connections

A. left (outer) join:

Left outer join (left join): the result set contains the matched rows in the connected table, and all rows in the left connected table.

SQL: select a. a, a. B, a. c, B. c, B. d, B. f from a left out join B ON a. a = B. c

B: right (outer) join:

Right outer join (right join): the result set includes both matched join rows in the connection table and all rows in the right join table.

C: full/cross (outer) join:

Full outer join: includes not only matching rows in the symbolic join table, but also all records in the two join tables.

12. Group: Group:

A table can only obtain group-related information after the query.

Group-related information: (statistical information) standard of count, sum, max, min, and avg groups)

When grouping in SQLServer: Fields of the text, ntext, and image types cannot be used as grouping bases.

Fields in the selecte statistical function cannot be put together with common fields;

13. perform operations on the database:

Detaching a database: sp_detach_db; appending a database: sp_attach_db indicates that the complete path name must be appended.

14. how to modify the database name:

Sp_renamedb 'old _ name', 'New _ name'

II. improvement

1. description: copy a table (only copy structure, source table name: a new table name: B) (Access available)

Method 1: select * into B from a where 1 <> 1 (for SQlServer only)

Method 2: select top 0 * into B from

2. description: copy a table (copy data, source table name: a target table name: B) (Access available)

Insert into B (a, B, c) select d, e, f from B;

3. description: table copying across databases (absolute path for specific data) (Access is available)

Insert into B (a, B, c) select d, e, f from B in 'specific database' where condition

Example:... from B in '"& Server. MapPath (". ") &"/data. mdb "&" 'Where ..

4. description: subquery (table name 1: table a name 2: B)

Select a, B, c from a where a IN (select d from B) or: select a, B, c from a where a IN (1, 2, 3)

5. description: displays the article, submitter, and last reply time.

Select a. title, a. username, B. adddate from table a, (select max (adddate) adddate from table where table. title = a. title) B

6. description: external join query (table name 1: table a name 2: B)

Select a. a, a. B, a. c, B. c, B. d, B. f from a left out join B ON a. a = B. c

7. description: online view query (table name 1:)

Select * from (SELECT a, B, c FROM a) T where t. a> 1;

8. description: between usage. when between restricts the Data Query range, it includes the boundary value. not between does not include

Select * from table1 where time between time1 and time2

Select a, B, c, from table1 where a not between value 1 and value 2

9. description: how to use in

Select * from table1 where a [not] in ('value 1', 'value 2', 'value 4', 'value 6 ')

10. description: two associated tables are used to delete information that is not in the secondary table.

Delete from table1 where not exists (select * from table2 where table1.field1 = table2.field1)

11. NOTES: Four table join query problems:

Select * from a left inner join B on. a = B. B right inner join c on. a = c. c inner join d on. a = d. d where .....

12. note: five minutes ahead of schedule reminder

SQL: select * from schedule where datediff ('Minute ', f start time, getdate ()> 5

13. note: one SQL statement is used to handle database paging.

Select top 10 B. * from (select top 20 primary Key Field, sorting field from table name order by sorting field desc) a, table name B where B. primary key field =. primary key field order by. sorting field

Specific implementation:

About database paging:

Declare @ start int, @ end int

@ SQL nvarchar (600)

Set @ SQL = 'SELECT top '+ str (@ end-@ start + 1) +' + from T where rid not in (select top '+ str (@ STR-1) + 'rid from T where Rid>-1 )'

Exec sp_executesql @ SQL

Note: a variable cannot be directly followed after top, so this is the only special method in actual application. Rid is an identifier column. if there are specific fields after top, this is very beneficial. This avoids the inconsistency in the actual table after the query results of the top field if it is a logical index. (the data in the logical index may be inconsistent with that in the data table, if the query is indexed, the index is first queried)

14. note: the first 10 records

Select top 10 * form table1 where range

15. Note: Select all the information of the largest record of a corresponding to the data with the same B value in each group (similar usage can be used for the monthly ranking of the Forum and analysis of popular products each month, rank by subject score, etc .)

Select a, B, c from tablename ta where a = (select max (a) from tablename tb where tb. B = ta. B)

16. description: Includes all rows in TableA but not in TableB and TableC and removes all repeated rows to derive a result table.

(Select a from tableA) before t (select a from tableB) before t (select a from tableC)

17. description: 10 data records are randomly taken out.

Select top 10 * from tablename order by newid ()

18. description: randomly selected records

Select newid ()

19. note: delete duplicate records

1), delete from tablename where id not in (select max (id) from tablename group by col1, col2 ,...)

2), select distinct * into temp from tablename

Delete from tablename

Insert into tablename select * from temp

Rating: this operation involves the movement of a large amount of data, which is not suitable for large-capacity but data operations.

3) for example, if you import data to an external table for the first time for some reasons, but it is difficult to determine the specific position, so that only the next import is complete, in this way, many repeated fields are generated. how to delete repeated fields?

Alter table tablename

-- Add an auto-increment column

Add column_ B int identity (1, 1)

Delete from tablename where column_ B not in (

Select max (column_ B) from tablename group by column1, column2 ,...)

Alter table tablename drop column column_ B

20. description: lists all table names in the database.

Select name from sysobjects where type = 'U' // U indicates the user

21. description: list all column names in the table.

Select name from syscolumns where id = object_id ('tablename ')

22. description: lists the fields of type, vender, and pcs, which are arranged by the type field. case can be easily selected, similar to case in select.

Select type, sum (case vender when 'a then pcs else 0 end), sum (case vender when 'C' then pcs else 0 end ), sum (case vender when 'B' then pcs else 0 end) FROM tablename group by type

Display result:

Type vender pcs

Computer A 1

Computer A 1

Cd B 2

Cd a 2

Mobile phone B 3

Mobile phone C 3

23. description: initialize table 1.

Truncate table table1

24. description: Select a record from 10 to 15.

Select top 5 * from (select top 15 * from table order by id asc) table _ alias order by id desc

III. skills

1. 1 = 2. it is used in combination of SQL statements.

"Where 1 = 1" indicates that all "where 1 = 2" are not selected,

For example:

If @ strWhere! =''

Begin

Set @ strSQL = 'SELECT count (*) as Total from ['+ @ tblName +'] where' + @ strWhere

End

Else

Begin

Set @ strSQL = 'SELECT count (*) as Total from ['+ @ tblName +']'

End

We can directly write

Error! Directory item not found.

Set @ strSQL = 'SELECT count (*) as Total from ['+ @ tblName +'] where 1 = 1 Anding '+ @ strWhere 2. shrink the database

-- Re-indexing

DBCC REINDEX

DBCC INDEXDEFRAG

-- Shrink data and logs

DBCC SHRINKDB

DBCC SHRINKFILE

3. compress the database

Dbcc shrinkdatabase (dbname)

4. transfer the database to a new user with the existing user permission

Exec sp_change_users_login 'update _ one', 'newname', 'oldname'

Go

5. check the backup set

Restore verifyonly from disk = 'E:/dvbbs. bak'

6. restore the database

Alter database [dvbbs] SET SINGLE_USER

GO

Dbcc checkdb ('dvbbs ', repair_allow_data_loss) WITH TABLOCK

GO

Alter database [dvbbs] SET MULTI_USER

GO

7. log clearing

SET NOCOUNT ON

DECLARE @ LogicalFileName sysname,

@ MaxMinutes INT,

@ NewSize INT

USE tablename -- Name of the database to be operated

SELECT @ LogicalFileName = 'tablename _ log', -- log file name

@ MaxMinutes = 10, -- Limit on time allowed to wrap log.

@ NewSize = 1 -- the size of the log file you want to set (M)

Setup/initialize

DECLARE @ OriginalSize int

SELECT @ OriginalSize = size

FROM sysfiles

WHERE name = @ LogicalFileName

SELECT 'original Size of '+ db_name () + 'log is' +

CONVERT (VARCHAR (30), @ OriginalSize) + '8 K pages or '+

CONVERT (VARCHAR (30), (@ OriginalSize * 8/1024) + 'mb'

FROM sysfiles

WHERE name = @ LogicalFileName

Create table DummyTrans

(DummyColumn char (8000) not null)

DECLARE @ Counter INT,

@ StartTime DATETIME,

@ TruncLog VARCHAR (255)

SELECT @ StartTime = GETDATE (),

@ TruncLog = 'backup log' + db_name () + 'WITH TRUNCATE_ONLY'

Dbcc shrinkfile (@ LogicalFileName, @ NewSize)

EXEC (@ TruncLog)

-- Wrap the log if necessary.

WHILE @ MaxMinutes> DATEDIFF (mi, @ StartTime, GETDATE () -- time has not expired

AND @ OriginalSize = (SELECT size FROM sysfiles WHERE name = @ LogicalFileName)

AND (@ OriginalSize * 8/1024)> @ NewSize

BEGIN -- Outer loop.

SELECT @ Counter = 0

WHILE (@ Counter <@ OriginalSize/16) AND (@ counter< 50000 ))

BEGIN -- update

INSERT DummyTrans VALUES ('fill log') DELETE DummyTrans

SELECT @ Counter = @ Counter + 1

END

EXEC (@ TruncLog)

END

SELECT 'final Size of '+ db_name () + 'log is' +

CONVERT (VARCHAR (30), size) + '8 K pages or '+

CONVERT (VARCHAR (30), (size * 8/1024) + 'mb'

FROM sysfiles

WHERE name = @ LogicalFileName

Drop table DummyTrans

SET NOCOUNT OFF

8. description: modify a table.

Exec sp_changeobjectowner 'tablename', 'dbo'

9. store and change all tables

Create procedure dbo. User_ChangeObjectOwnerBatch

@ OldOwner as NVARCHAR (128 ),

@ NewOwner as NVARCHAR (128)

AS

DECLARE @ Name as NVARCHAR (128)

DECLARE @ Owner as NVARCHAR (128)

DECLARE @ OwnerName as NVARCHAR (128)

DECLARE curObject CURSOR

Select 'name' = Name,

'Owner' = user_name (uid)

From sysobjects

Where user_name (uid) = @ OldOwner

Order by name

OPEN curObject

Fetch next from curObject INTO @ Name, @ Owner

WHILE (@ FETCH_STATUS = 0)

BEGIN

If @ Owner = @ OldOwner

Begin

Set @ OwnerName = @ OldOwner + '.' + rtrim (@ Name)

Exec sp_changeobjectowner @ OwnerName, @ NewOwner

End

-- Select @ name, @ NewOwner, @ OldOwner

Fetch next from curObject INTO @ Name, @ Owner

END

Close curObject

Deallocate curObject

GO

10. data is written cyclically in SQL SERVER.

Declare @ I int

Set @ I = 1

While @ I <30

Begin

Insert into test (userid) values (@ I)

Set @ I = @ I + 1

End

Case:

As shown in the following table, all