To add a composite primary key

Steps to add a composite primary key

Composite primary key ： There are multiple primary keys in a table

At this point, you need to add a new primary key , Compound primary keys can only be added at the same time , Create at the same time , Adding directly will report an error

# Delete all primary keys 
ALTER TABLE  Table name  DROP PRIMARY KEY:
# Add new primary key field 
ALTER TABLE  Table name  ADD
 Field name  VARCHAR (1) COMMENT ' remarks xxx' NOT NULI DEFAULT
# Add primary key 
ALTER TABLE  Table name  ADD PRIMARY KEY(
` Field name 1`,
` Field name 2`,
` Field name 3`,
` Field name 4`
)

Add to complete ！