Basic SQL knowledge

 

Basic SQL knowledge
1. database query: Select usage
Select [top (numeric)] field list from data table [Where condition] [order by field] [ASC or DESC]

2. Add data: insert into usage
Insert into data table (Field 1, Field 2, Field 3 ,...) Values (value of Field 1, value of Field 2, value of Field 3 ,...)

3. delete data: delete usage
Delete from data table [Where condition]

4. Update Data: Update usage
Update data table set field 1 = field value 1, Field 2 = field value 2 ,... [Where condition]

5. Create a data table: Create Table usage
Create Table data table name (Field 1 Name, Field 1 type, Field 2 Name field 2 type ,...)

Common Data Types of access data tables: Text, char (number), memo, number, Int, date/time, logical, oleobject

Example of adding an automatic number field: Create Table AAA (ID int identity (1, 1) not null, ABC varchar (25) null)

6. Change the data table: alter table usage
Add field: alter table data table name Add column field Name field type

Delete field: alter table data table name drop column field name

7. delete a data table: Drop table usage

Drop table data table name

 

********************

Alter table [Table] alter [ID] counter constraint [Table _ p] primary key: Change the ID column to the automatic ID type and set it to the primary key.

-- Sentence skill
-- Data Operations
Select -- retrieve data rows and columns from database tables
Insert -- add new data rows to the database table
Delete -- delete data rows from a database table
Update -- update data in the database table
-- Data Definition
Create Table -- create a database table
Drop table -- delete a table from a database
Alter table -- modify the database table structure
Create View -- create a view
Drop View -- delete a view from a database
Create index -- create an index for the database table
Drop index -- delete an index from a database
Create procedure -- create a stored procedure
Drop procedure -- delete a stored procedure from a database
Create trigger -- create a trigger
Drop trigger -- delete a trigger from a database
Create schema -- Add a new schema to the database
Drop schema -- delete a schema from the database
Create Domain -- create a Data Value Domain
Alter Domain -- change domain definition
Drop Domain -- delete a domain from the database
-- Data Control
Grant -- Grant the user access permission
Deny-Deny User Access
Revoke -- revoke User Access Permissions
-- Transaction control
Commit -- end the current transaction
Rollback -- abort the current transaction
SET transaction -- define the data access features of the current transaction
-- Programmatic SQL
Declare -- set the cursor for the query
Explain -- describe the data access plan for query
Open -- open a cursor for retrieving query results
Fetch -- retrieve a row of query results
Close -- close the cursor
Prepare -- prepare SQL statements for dynamic execution
Execute -- dynamically Execute SQL statements
Describe -- describe the prepared Query

--- Local variables
Declare @ ID char (10)
-- Set @ ID = '20140901'
Select @ ID = '000000'

--- Global variables
--- It must start @

-- If else
Declare @ x int @ Y int @ Z int
Select @ x = 1 @ Y = 2 @ z = 3
If @ x> @ Y
Print 'x> y' -- print the string 'x> y'
Else if @ Y> @ Z
Print 'y> Z'
Else print 'z> y'

-- Case
Use pangu
Update employee
Set e_wage =
Case
When job_level = '1' then e_wage * 1.08
When job_level = '2' then e_wage * 1.07
When job_level = '3' then e_wage * 1.06
Else e_wage * 1.05
End

-- While continue break
Declare @ x int @ Y int @ C int
Select @ x = 1 @ Y = 1
While @ x <3
Begin
Print @ X -- print the value of variable X
While @ Y <3
Begin
Select @ C = 100 * @ x + @ Y
Print @ C -- print the value of variable C
Select @ Y = @ Y + 1
End
Select @ x = @ x + 1
Select @ Y = 1
End

-- Waitfor
-- For example, the SELECT statement is executed after 1 hour, 2 minutes, and 3 seconds.
Waitfor delay '01: 02: 03'
Select * from employee
-- For example, the SELECT statement will not be executed until PM.
Waitfor time '23: 08: 00'
Select * from employee

 

* ** Select ***

Select * (column name) from table_name (Table Name) Where column_name operator Value
Ex: (host)
Select * From stock_information where stockid = STR (NID)
Stockname = 'str _ name'
Stockname like '% find this %'
Stockname like '[A-Za-Z] %' --------- ([] specifies the range of values)
Stockname like '[^ F-M] %' --------- (^ exclude specified range)
--------- Only wildcard characters can be used in the WHERE clause that uses the like keyword)
Or stockpath = 'stock _ Path'
Or stocknumber: <1000
And stockindex = 24
Not stocksex = 'man'
Stocknumber between 20 and 100
Stocknumber in (10, 20, 30)
Order by stockid DESC (ASC) --------- sort, desc-descending, ASC-ascending
Order by 1, 2 --------- by column number
Stockname = (select stockname from stock_information where stockid = 4)
--------- Subquery
--------- Unless the inner select clause can only return the value of one row,
--------- Otherwise, an in qualifier should be used in the outer WHERE clause.
Select distinct column_name form table_name --------- distinct specifies the unique column value to be retrieved, not repeated
Select stocknumber, "stocknumber + 10" = stocknumber + 10 from table_name
Select stockname, "stocknumber" = count (*) from table_name group by stockname
--------- Group by groups the table by row. The specified column has the same value.
Having count (*) = 2 --------- having: select the specified group.

Select *
From Table1, Table2
Where table1.id * = table2.id -------- left external connection, which exists in Table1 but not in Table2.
Table1.id = * table2.id -------- right external connection

Select stockname from Table1
Union [all] ----- Union merges query result sets, all-retained duplicate rows
Select stockname from Table2

* ** Insert ***

Insert into table_name (stock_name, stock_number) value ("XXX", "XXXX ")
Value (select stockname, stocknumber from stock_table2) --- value is a SELECT statement

* ** Update ***

Update table_name set stockname = "XXX" [where stockid = 3]
Stockname = default
Stockname = NULL
Stocknumber = stockname + 4

* ** Delete ***

Delete from table_name where stockid = 3
Truncate table_name ----------- delete all rows in the table and maintain table integrity
Drop table table_name --------------- delete a table completely

* ** Alter table ** --- modify the database table structure

Alter table database. Owner. table_name add column_name char (2) null .....
Sp_help table_name ---- display existing features of a table
Create Table table_name (name char (20), age smallint, lname varchar (30 ))
Insert into table_name select ...... ----- to delete a column (create a new table)
Alter table table_name drop constraint stockname_default ---- Delete the default constraint of stockname

* ** Function (/* common functions */)***

---- Statistical functions ----
AVG -- average value
Count -- count
Max -- calculate the maximum value
Min -- Minimum value
Sum -- sum

-- AVG
Use pangu
Select AVG (e_wage) as dept_avgwage
From employee
Group by dept_id

-- Max
-- Name of the employee with the highest salary
Use pangu
Select e_name
From employee
Where e_wage =
(Select max (e_wage)
From employee)

-- STDev ()
-- STDev () function returns the standard deviation of all data in the expression.

-- Stdevp ()
-- The stdevp () function returns the population standard deviation.

-- VAR ()
-- The VaR () function returns the statistical variation of all values in the expression.

-- Varp ()
-- Varp () function returns the total number of variations

---- Arithmetic functions ----

/*** Trigonometric function ***/
Sin (float_expression) -- returns the sine of the angle in radians
Cos (float_expression) -- returns the cosine of the angle in radians
Tan (float_expression) -- returns the tangent of the angle in radians
Cot (float_expression) -- returns the cotangent of the angle in radians
/*** Inverse trigonometric function ***/
Asin (float_expression) -- returns the angle in radians where the sine is the float value.
ACOs (float_expression) -- returns the angle in radians where the cosine is the float value.
Atan (float_expression) -- returns the angle in radians of the float value.
Atan2 (float_expression1, float_expression2)
-- Returns the radians of float_expression1/float_expres-sion2.
Degrees (numeric_expression)
-- Converts radians to degrees and returns data of the same type as the expressions.
-- Integer/money/real/Float Type
Radians (numeric_expression) -- converts degrees to radians and returns the same data type as the expression.
-- Integer/money/real/Float Type
Exp (float_expression) -- returns the exponent value of the expression.
Log (float_expression) -- returns the natural logarithm of the expression.
Log10 (float_expression) -- returns the base-10 logarithm of the expression.
SQRT (float_expression) -- returns the square root of the expression
/***** Approximate function ***/
Ceiling (numeric_expression) -- Return> = the minimum integer of the expression. The returned data type is the same as that of the expression.
-- Integer/money/real/Float Type
Floor (numeric_expression) -- returns the minimum integer of the <= expression. The returned data type is the same as that of the expression.
-- Integer/money/real/Float Type
Round (numeric_expression) -- returns the data returned from a rounding value with integer_expression as the precision.
-- The same type and expression can be integer/money/real/Float Type
ABS (numeric_expression) -- returns the absolute value of the expression. The returned data type is the same as that of the expression.
-- Integer/money/real/Float Type
Sign (numeric_expression) -- the positive and negative values of the test parameter return the data type returned by 0, zero, 1, positive, or-1 negative.
-- Same as the expression, it can be of the integer, money, real, or float type.
Pi () -- the return value is π, that is, 3.1415926535897936
Rand ([integer_expression]) -- use the optional [integer_expression] As the seed value to obtain a random floating point number between 0 and 1.

---- String function ----
ASCII () -- returns the ASCII value of the leftmost character of a character expression.
Char () -- function used to convert ASCII code to character
-- If no value is entered ~ The Char function returns a null value for the ASCII value between 255.
Lower () -- function converts all strings to lowercase letters
Upper () -- function converts all strings to uppercase
STR () -- function converts numeric data to numeric data
Ltrim () -- function removes spaces in the string Header
Rtrim () -- function removes spaces at the end of the string
Left (), right (), substring () -- The function returns some strings.
Charindex (), patindex () -- The function returns the starting position of a specified substring in the string.
Soundex () -- The function returns a four-digit escape code.
-- The soundex function can be used to find strings with similar sound. However, the soundex function returns only 0 values for numbers and Chinese characters.
Difference () -- the difference between the two character expressions returned by the soundex Function
-- 0 the first character returned by the two soundex functions is different
-- 1 the first character returned by the two soundex functions is the same
-- 2 two soundex functions return the same first and second characters.
-- 3 two soundex functions return the same first, second, and third characters.
-- 4 two soundex functions return identical values

Quotename () -- The function returns a string enclosed by a specific character.
/* Select quotename ('abc', '{') quotename ('abc ')
The running result is as follows:
----------------------------------{
{ABC} [ABC] */

Replicate () -- The function returns a string that repeats character_expression for a specified number of times.
/* Select replicate ('abc', 3) replicate ('abc',-2)
The running result is as follows:
----------------------
Abcabcabc null */

Reverse () -- The function reverses the character arrangement order of the specified string
Replace () -- returns the string with the specified substring replaced by the function.
/* Select Replace ('abc123g ', '123', 'def ')
The running result is as follows:
----------------------
Abcdefg */

Space () -- The function returns a blank string with a specified length.
Stuff () -- The function replaces a substring of the specified position length with another substring.

---- Data type conversion function ----
The syntax of the cast () function is as follows:
Cast () (<expression> as <data _ type> [length])
The convert () function syntax is as follows:
Convert () (<data _ type> [length], <expression> [, style])

Select cast (100 + 99 as char) convert (varchar (12), getdate ())
The running result is as follows:
------------------------------------------
199 Jan 15 2000

---- Date function ----
Day () -- The function returns the date value in date_expression.
Month () -- The function returns the month value in date_expression.
Year () -- returns the Year Value in date_expression.
Dateadd (<datepart>, <number>, <date>)
-- The function returns the new date generated by the specified date plus the specified additional date interval number.
Datediff (<datepart >,< number >,< date>)
-- The function returns the datepart differences between two specified dates.
Datename (<datepart>, <date>) -- The function returns the specified part of the date in the form of a string.
Datepart (<datepart >,< date>) -- The function returns the specified part of the date in the form of an integer.
Getdate () -- The function returns the current date and time of the system in the default format of datetime.

---- System functions ----
App_name () -- function returns the name of the currently executed application
Coalesce () -- function returns the value of the first non-null expression among multiple expressions.
Col_length (<'table _ name' >,< 'column _ name'>) -- returns the length value of the specified field in the table.
Col_name (<table_id >,< column_id>) -- the name of the specified field in the function return table is the column name.
Datalength () -- the actual length of the data returned by the function
Db_id (['database _ name']) -- number of the database returned by the function
Db_name (database_id) -- Name of the database returned by the function
Host_id () -- The function returns the name of the server computer.
Host_name () -- returns the name of the server computer.
Identity (<data_type> [, seed increment]) [as column_name])
-- The identity () function is used only in the select into statement to insert an identity column to the new table.
/* Select Identity (INT, 1, 1) as column_name
Into newtable
From oldtable */
Isdate () -- determines whether the given expression is a reasonable date.
Isnull (<check_expression >,< replacement_value>) -- The function replaces the null value in the expression with the specified value.
Isnumeric () -- The function determines whether the given expression is a reasonable value.
Newid () -- The function returns a value of the uniqueidentifier type.
Nullif (<expression1>, <expression2>)
-- The nullif function returns the value of expression1 if expression1 is equal to expression2. If the value is not equal, the return value is expression1.