MySQL learning summary

Three paradigms of database

• First normal form ： No more columns can be split
• Second normal form ： On the basis of the first paradigm , Non primary key columns are completely dependent on the primary key
• Third normal form ： In the second paradigm , Non primary key columns only depend on primary keys , Do not rely on other non primary keys

Concurrency of transactions

• Dirty reading ： Business A Read transaction B Updated data , Then the data B Roll back , that A Read dirty data
• It can't be read repeatedly ： Business A Read the same data multiple times , Business B In the transaction A In the process of reading, the data is modified and submitted , Lead to A The data read many times is different
• Fantasy reading ： Business A When reading data , Business B Added data , Lead to A After reading, I found that there was still data I didn't read
• Unrepeatable reading focuses on revision , Unreal reading focuses on adding or deleting
• To solve the problem of unrepeatable reading, you only need to lock the line , To solve phantom reading, you need to lock the table

The isolation level of the transaction

Transaction isolation level	Dirty reading	It can't be read repeatedly
Read uncommitted	yes	yes
It can't be read repeatedly	no	yes
Repeatable	no	no
Serialization	no	no

b-tree、b+tree The difference between

• b-tree Key words of 、 Indexes and records are put together
• b+tree Records of are only placed in leaf nodes , Instead of leaf nodes, there are only keywords and indexes pointing to the next node
• b-tree The closer to the root node, the faster the record lookup event , Just find the keyword to confirm the existence of the record
• b+tree The search events of each record in are basically the same , You need to go from the root node to the leaf node
• b+tree The cost of disk reading and writing is lower , Query efficiency is more stable ,b+tree More suitable for indexing

drop、delete、truncate The difference between

• Use it directly when you don't need another table drop Delete
• Delete all data , But keep the table structure with truncate
• Use when you want to delete some data delete

union and union all The difference between

• union It will get more data for de duplication and sorting
• union all All query results will be preserved , There will be no de duplication and sorting
• Without losing weight ,union all It's more efficient

char and varchar The difference between

• char, Fixed length , Fixed length strings are recommended char Type storage , Space saving and efficient
• varchar, Indefinite length , Low efficiency

Why should a field be defined as not null

• null Values take up more bytes , And it will cause many unexpected situations in the program