Bi SQL drop & alter

Drop & Alter

Drop, Cancel the operation ;Alter, update operation .

stay PowerBI in , If we modify some data cleaning operations , Just open the advanced editor to view M Language , You can modify and delete specific statements .

But in the database warehouse , We often need the help of Drop and Alter Or other statements to modify the data warehouse .

Why can't we be in a data warehouse like PowerQuery Do the same in ？

In fact, this is related to the positioning of the two .

PowerQuery The positioning of , Is aimed at PowerView To serve , although PowerQuery Self system , It can be called a mini data warehouse , But in general , The data we get from the standard data warehouse are relatively clean data .

SQL Servers The positioning of , It is one of the international basic database languages , Its positioning is a large data warehouse . In the standardized standards of data governance , The data warehouse must be stable 、 Continuously provide data source .

therefore , For the sake of database stability , If we need to modify and adjust the logic of the data warehouse , There must be a set of standardized operation processes and specialized logic modification statements .

grammar

Drop grammar ：

1. Delete index ：

DROP INDEX  The index name  ON  The name of the table 

2. Delete table ：

DROP TABLE  The name of the table 

notes ： This operation , Will delete the structure of the table 、 Properties and index ！

3. Delete database ：

DROP DATABASE  Database name 

4. Keep the table structure , Delete data only ：

TRUNCATE TABLE  The name of the table 

Alter grammar ：

1. Add columns to the table ：

ALTER TABLE  The name of the table 
ADD  Column name   data type 

2. Delete the column in the table ：

ALTER TABLE  The name of the table 
DROP COLUMN  Column name 

3. Change the data type of the column in the table ：

ALTER TABLE  The name of the table 
ALTER COLUMN  Column name   data type 

Constraints Add

White tea has shared several types of constraints before , In fact, there were additional Drop and Alter Operation of the , Considering the classification problem , White tea decided to put it in this issue .

All the following syntax , When the table has been created , We need to make changes to the constraints .

• UNIQUE（ only ） to update ： Add unique constraint ：

ALTER TABLE  The name of the table 
ADD UNIQUE ( Column name )

Add multiple column unique constraints ：

ALTER TABLE  The name of the table 
ADD CONSTRAINT uc_ Constraint name  UNIQUE ( Column name , Column name ...)

Undo unique constraint ：

ALTER TABLE  The name of the table 
DROP CONSTRAINT uc_ Constraint name 

• PRIMARY KEY（ Primary key ） to update ：

Add a primary key constraint ：

ALTER TABLE  The name of the table 
ADD PRIMARY KEY ( Column name )

Add multiple column primary key constraints ：

ALTER TABLE  The name of the table 
ADD CONSTRAINT pk_ Constraint name  PRIMARY KEY ( Column name , Column name ...)

Undo the primary key constraint ：

ALTER TABLE  The name of the table 
DROP CONSTRAINT pk_ Constraint name 

• FOREIGN KEY（ Foreign keys ） to update ：

New foreign key constraint ：

ALTER TABLE  The name of the table 
ADD FOREIGN KEY ( Column name )
REFERENCES  Primary key table ( Column name )

Add multiple column foreign key constraints ：

ALTER TABLE  The name of the table 
ADD CONSTRAINT fk_ Constraint name 
FOREIGN KEY ( Column name )
REFERENCES  Primary key table ( Column name )

Undo the foreign key constraint ：

ALTER TABLE  The name of the table 
DROP CONSTRAINT fk_ Constraint name 

• CHECK（ Check ） to update ：

Add check constraint ：

ALTER TABLE  The name of the table 
ADD CHECK ( Column name   Comparison symbol  " Conditions ")

Add multi column check constraint ：

ALTER TABLE  The name of the table 
ADD CONSTRAINT chk_ Constraint name  CHECK ( Column name   Comparison symbol  " Conditions " AND  Column name   Comparison symbol  " Conditions " ......)

Undo check constraint ：

ALTER TABLE  The name of the table 
DROP CONSTRAINT chk_ Constraint name 

• DEFAULT（ The default value is ） to update ：

New default value constraint ：

ALTER TABLE  The name of the table 
ALTER COLUMN  Column name  SET DEFAULT ' The default value is '

Undo the default value constraint ：

ALTER TABLE  The name of the table 
ALTER COLUMN  Column name  DROP DEFAULT

That's the end of this issue , No use cases , Because we do BI In fact, it is rare for the front end to use such operations , We are more simply using query aggregation statements .

Of course , Yes ETL And interested partners of the data warehouse can have a try .

This is white tea , One PowerBI Beginners .