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Databases Cheatsheet

Common

SQL style guide

http://www.sqlstyle.guide/ru/

PostgreSQL

useful queries

  • check the current session TZ
   SELECT current_setting('TIMEZONE');
   show timezone;
  • show db tables
   SELECT * FROM pg_catalog.pg_tables;
  • show tables without system tables
   SELECT * FROM pg_catalog.pg_tables WHERE schemaname != 'pg_catalog' AND schemaname != 'information_schema';
  • list all databases
  • \l
  • show tables \tables and sequences \tables and views
  • \dt \d \dS
  • use database
  • \c dbname
  • alter column type e.g. boolean to integer
   ALTER TABLE tbname ALTER colname SET DEFAULT null;

   ALTER TABLE tbname ALTER colname TYPE INTEGER USING CASE WHEN colname = false THEN 0 ELSE 1 END;

   ALTER TABLE tbname ALTER colname SET DEFAULT 0;
   COMMIT;

backup, export database

  • whole
  • pg_dump -U username -f /path/dump.sql dbname
  • schema only
  • pg_dump -U username -s -f /path/dump.sql dbname
  • single table
  • pg_dump -U username -d dbname -t tbname > dump.sql
  • single table data only
  • pg_dump -U username --data-only --table=tablename sourcedb > dump.sql
  • export data to csv
   COPY (SELECT * FROM table1) TO '/path/to/csv/data.txt' (format CSV);
   COPY (SELECT * FROM table1) TO '/path/to/csv/data.txt' WITH CSV DELIMITER ';' FORCE QUOTE *;

restore, import database

  • restore database
  • \i /path/dump.sql
  • single table to database
  • psql dbname < dump.sql
  • extracting single table out of the large dump
  • pg_restore --data-only --table=tablename fulldump.pg > dump.pg
  • import data from csv
   COPY table1(col1, col2, col3) FROM '/path/to/csv/data.txt' WITH (FORMAT csv);
  • import data from csv - include the header row
   COPY table1 FROM '/path/to/csv/data.txt' DELIMITER ',' CSV HEADER;

useful system commands

  • show pg_config file location
  SHOW config_file

  • connect as admin

  • psql -h localhost postgres postgres

  • prevent future connections && terminate all connections to this db except your own

  REVOKE CONNECT ON DATABASE thedb FROM public;
  SELECT pid, pg_terminate_backend(pid) FROM pg_stat_activity WHERE datname = current_database() AND pid <> pg_backend_pid();
  GRANT CONNECT ON DATABASE thedb TO public;
  • drop all active sessions
  ALTER DATABASE db CONNECTION LIMIT 1;
  SELECT pg_terminate_backend(pid) FROM pg_stat_activity WHERE datname = 'db';

MySQL

restore, import database

  • import table with the same name from one database to another
   CREATE TABLE db1.table1 SELECT * FROM db2.table1;
  • import data from csv file. Basename of a file must be equal to tablename
  • mysqlimport -u user -p -c column1,column2 --fields-terminated-by=; database_name table_name.txt

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