Database engines in practice don’t actually run queries by joining, and then filtering, and then grouping, because they implement a bunch of optimizations reorder things to make the query run faster as long as reordering things won’t change the results of the query

SELECT isn’t the first thing, it’s like the 5th thing!

DBA Por Acaso: RDS, MySQL e Tuning para Iniciantes

SELECT sj.name , sja.* FROM msdb.dbo.sysjobactivity AS sja INNER JOIN msdb.dbo.sysjobs AS sj ON sja.job_id = sj.job_id WHERE sja.start_execution_date IS NOT NULL AND sja.stop_execution_date IS NULL

sys.database_mirroring

ALTER DATABASE dbName SET PARTNER TIMEOUT 20

ALTER DATABASE SET trustworthy on

n the Advanced Security Settings dialog box, make sure that SELF is listed under Permission entries. If SELF is not listed, click Add, and then add SELF.Under Permission entries, click SELF, and then click Edit.In the Permission Entry dialog box, click the Properties tab.On the Properties tab, click This object only in the Apply onto list, and then click to select the check boxes for the following permissions under Permissions:Read servicePrincipalNameWrite servicePrincipalName

rant delegation permission to the SQL Server service account domain user account.

The client and server computers must be part of the same Windows domain, or in trusted domains. A Service Principal Name (SPN) must be registered with Active Directory, which assumes the role of the Key Distribution Center in a Windows domain. The SPN, after it is registered, maps to the Windows account that started the SQL Server instance service. If the SPN registration has not been performed or fails, the Windows security layer cannot determine the account associated with the SPN, and Kerberos authentication will not be used.

with recursive rnd_move(move) as ( select *, random() rnd from generate_series(1, 9) move ), winning_positions(a, b, c) as ( values (1, 2, 3), (4, 5, 6), (7, 8, 9), -- rows (1, 4, 7), (2, 5, 8), (3, 6, 9), -- cols (1, 5, 9), (3, 5, 7) -- diagonals ), game as ( select 'O' as who_next, ARRAY['.', '.', '.', '.', '.', '.', '.', '.', '.'] as board union ( select case when who_next = 'X' then 'O' else 'X' end as who_next, board[:move-1] || who_next || board[move+1:] from game, rnd_move where board[move] = '.' order by rnd limit 1 ) ), game_with_winner as ( select *, lag(a is not null) over () as finished, lag(who_next) over () as who from game left join winning_positions on board[a] != '.' and board[a] = board[b] and board[a] = board[c] ) select array_to_string(board[1:3] || chr(10) || board[4:6] || chr(10) || board[7:9] || chr(10), '') board, case when a is not null then who || ' wins' end as winner from game_with_winner where not finished;

sqlite> .mode column sqlite> .headers on

select top 1 * from newsletters where IsActive = 1 order by PublishDate desc

PostgreSQL連接Python

REQUIRED fields are no longer supported in Standard SQL. If you're using Standard SQL (as opposed to Legacy SQL), they recommend you change all your REQUIRED fields to NULLABLE.

significant.

Todo o dado (valor atómico) pode ser acedido logicamente (e unicamente) usando o nome da tabela, o valor da chave primária da linha e o nome da coluna

Um banco de dados relacional é um banco de dados que modela os dados de uma forma que eles sejam percebidos pelo usuário como tabelas, ou mais formalmente relações.

chaves estrangeiras

chaves candidatas

Uma relação é um conjunto desordenado de tuplas.

Na construção da tabela identificam-se os dados da entidade. A atribuição de valores a uma entidade constrói um registro da tabela.

conjunto de pares ordenados de domínio e nome que serve como um cabeçalho para uma relação.

Os blocos básicos do modelo relacional são o domínio, ou tipo de dado.

Uma relação é similar ao conceito de tabela e uma tupla é similar ao conceito de linha.

Num banco de dados relacional, quando um registro aponta para o outro, dependente deste, há de se fazer regras para que o registro "pai" não possa ser excluído se ele tiver "filhos" (as suas dependências).

SqlResultSetMapping

2.3.2. Mapping native queries You can also map a native query (ie a plain SQL query). To achieve that, you need to describe the SQL resultset structure using @SqlResultSetMapping (or @SqlResultSetMappings if you plan to define several resulset mappings). Like @NamedQuery, a @SqlResultSetMapping can be defined at class level or in a JPA XML file. However its scope is global to the application.

@SqlResultSetMappings( { @SqlResultSetMapping(name = "ProfessorWithAddress", entities = { @EntityResult(entityClass = Professor.class), @EntityResult(entityClass = Address.class) }) })

Use @FieldResult in the SqlResultSetMapping, to link each entity property to its column alias