SQL regular expressions are a curious cross between LIKE notation and common (POSIX) regular expression notation.

The Realm is a new database module that is improving the way databases are used and also supports relationships between objects. If you are part of the SQL development world, then you must be familiar with the Realm.

[:returning] (Postgres-only) An array of attributes that should be returned for all successfully inserted records. For databases that support INSERT ... RETURNING, this will default to returning the primary keys of the successfully inserted records. Pass returning: %w[ id name ] to return the id and name of every successfully inserted record or pass returning: false to omit the clause.

Comparison Time … 🤞

NoSQL databases typically perform better and are easier to scale due to the nature of their data access and storage

WHERE clause introduces a condition on individual rows; HAVING clause introduces a condition on aggregations, i.e. results of selection where a single result, such as count, average, min, max, or sum, has been produced from multiple rows. Your query calls for a second kind of condition (i.e. a condition on an aggregation) hence HAVING works correctly. As a rule of thumb, use WHERE before GROUP BY and HAVING after GROUP BY. It is a rather primitive rule, but it is useful in more than 90% of the cases.

Which database technology to choose

1) Redash and Falcon focus on people that want to do visualizations on top of SQL2) Superset, Tableau and PowerBI focus on people that want to do visualizations with a UI3) Metabase and SeekTable focus on people that want to do quick analysis (they are the closest to an Excel replacement)

Joins are not expensive. Who said it to you? As basically the whole concept of relational databases revolve around joins (from a practical point of view), these product are very good at joining. The normal way of thinking is starting with properly normalized structures and going into fancy denormalizations and similar stuff when the performance really needs it on the reading side. JSON(B) and hstore (and EAV) are good for data with unknown structure.

Another nice SQL script paired with CRON jobs was the one that reminded people of carts that was left for more than 48 hours. Select from cart where state is not empty and last date is more than or equal to 48hrs.... Set this as a CRON that fires at 2AM everyday, period with less activity and traffic. People wake up to emails reminding them about their abandoned carts. Then sit watch magic happens. No AI/ML needed here. Just good 'ol SQL + Bash.

I will write a query like select from order table where last shop date is 3 or greater months. When we get this information, we will send a nice "we miss you, come back and here's X Naira voucher" email. The conversation rate for this one was always greater than 50%.

ACID stands for Atomicity (an operation either succeeds completely or fails, it does not leave partial data), Consistency (once an application performs an operation the results of that operation are visible to it in every subsequent operation), Isolation (an incomplete operation by one user does not cause unexpected side effects for other users), and Durability (once an operation is complete it will be preserved even in the face of machine or system failure).

The SELECT part should not contain any columns not referenced in GROUP BY clause, unless it is wrapped with an aggregate function.

Before SQL3 (1999), the selected fields must appear in the GROUP BY clause

Event.joins(:packages).having('array_agg(packages.type) @> array[?]', packages).group(:id)

Which to use? ANY is a later, more versatile addition, it can be combined with any binary operator returning a boolean value. IN burns down to a special case of ANY. In fact, its second form is rewritten internally: IN is rewritten with = ANY NOT IN is rewritten with <> ALL

Single quotes return text strings.  Double quotes return (if you can really think of them as “returning” anything) identifiers, but with the case preserved.

if you care at all about storing timestamps in MySQL, store them as integers and use the UNIX_TIMESTAMP() and FROM_UNIXTIME() functions.

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