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142 lines
4.0 KiB
Markdown
142 lines
4.0 KiB
Markdown
---
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title: "18-advanced-SQL"
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aliases:
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tags:
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- info201
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- lecture
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sr-due: 2022-05-15
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sr-interval: 3
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sr-ease: 250
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---
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varchar usually bigger than you think
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# CRUD
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- insert adds a row 
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- select retrieves rows from the table
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- ouput can be "saved" as a view 
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- changes to the underlying table also chagnes the view
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- update modifies rows 
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- delete removes rows  
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- test as a select statement first
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# SQL DAO programming
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We want to miniminse load on sql as connecting to database is expensive.
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Optimisations:
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- prefer muultiple row operations
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- connection pools (keep connections "alive")
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- reuse prepared statements (reduce unnecessary SQL parsing)
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- consider combining queries to reduce round trips
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- batched requests
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Follow Optimistic approach
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- assume operatios will succeed (no pre checking)
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- handle errors with exception handling
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- consider using merge if available
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## Merge
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 aka replace etc.
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- update if they exist other wise insert
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## JDBC
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Java framework for interacting with sql databases.
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plaform and DBMS independent
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- driver provided by DBMS vendors
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- same Java code will work with any DBMC
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key concepts
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- connections and connection pools
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- sql strings and prepared statements
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- result sets
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- transactions
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- batched requrests
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## JDBI
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- bette version of JDBI
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- layered on top of [JDBC](#JDBC)
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- better APIs
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- less code
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- simple class <-> table mapping
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- flexible plugin architecture
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# Transactions
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interaction between two entities
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- follow explicit or implied forms
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- usually involves exchange on resources
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- may require several steps
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- often considered a single unit
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## In data bases
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- group of db operations is considered a *single logical unit*
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- transfer (read and update)
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- recieve shipement (update accounts)
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- customer sale
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transactions are all or nothing. (commit vs rollback)
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## ACID
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- Atomic
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- all or nothing
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- operations should be related
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- Consistent
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- transactions move dbs from one consistent state to another
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- "consistent" ⇒ all integrity rules are satisfied
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- db may be inconsistent during a transaction
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- require defferable constraints
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- Isolated
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- concurrent transactios shouldn't interfere with each other
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- ideally behave as if other transactions dont exist
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- read committed isolation
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- uncommited changes are visible to other transactions
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- require some form of concrrency mangement (e.g., locking)
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- 
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- Durability
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- once a transaction is commited it is permanent
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- uncommited transaction dont survive crashes
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- auto rollback of uncommitted transaction
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## commit and rollback
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- changes are made to "live" data
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- commit makes database changes permanent
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- rollback removes all changes since that transaction start
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## Transaction in Java
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default to auto commit.
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- each statement is a separate transaction
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- if transaction has multiple statements
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- disable auto commit
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- you must manage commit and rollback yourself
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# Select
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- select <- wahat
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- from <- from where
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- where <- filter
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- group <- aggregation
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- order <- order
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distinct removes duplicate rows
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