If you’re writing functions in postgres then you’ll probably be using a language such as plpgsql. Let’s say you’re writing a script to to add all of these functions to a new database, but you don’t know whether that language has been created yet. You’ll probably want to do something like CREATE LANGUAGE IF NOT [...]
Posts Tagged ‘PostgreSQL’
CREATE LANGUAGE if it doesn’t exist in PostgreSQL
Posted: 27th August 2011 by Tim in PostgreSQLTags: create, create language, exist, if exists, if not exists, language, plpgsql, postgres, PostgreSQL, query, script, sql, statement
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Conditional COUNT in SQL
Posted: 10th October 2010 by Tim in SQLTags: count, COUNTIF, if, mssql, MySQL, oracle, PostgreSQL, query, sql, sql server, statement, sum
Sometimes you want to count the number of results which meet a certain condition. Excel has a COUNTIF(…) function but there is nothing like that in the (current) SQL standard. The solution is to mix the SUM function with a CASE statement. Confused? Let’s see an example. Imagine you have two tables – student (student_id, [...]
Concatenation in PostgreSQL
Posted: 15th June 2010 by Tim in PostgreSQLTags: concat, concatenation, database, PostgreSQL, result, server, SET, sql, string
It’s often useful to concatenate two fields together in an SQL query. In PostgreSQL, fields may be concatenated using the || operator. The syntax is really simple. Just place the double bar between the fields you want to join, and (optionally) give a label to that new field. Strings can also be concatenated with the [...]
Inline CASE statement in PostgreSQL
Posted: 9th June 2010 by Tim in PostgreSQLTags: case, database, if, PostgreSQL, query, select, server, sql, statement, switch
PostgreSQL, unfortunately, does not provide an inline IF statement like some other SQL servers. CASE statements, however, can be run inline which can be quite handy. Let’s say you have a user table with a num_heads field. You want to know if the user is a zombie, human or alien with one query. This could [...]
SHOW SCHEMAS in PostgreSQL
Posted: 18th February 2010 by Tim in PostgreSQLTags: display, postgres, PostgreSQL, psql, schema, schemas, show
If you’re looking for a command to give you a list of schemas in PostgreSQL, much like MySQL’s SHOW SCHEMAS command, then \dn is what you’re looking for. ie: \dn List of schemas Name | Owner ——————-+——— information_schema | postgres pg_catalog | postgres person | tim …
Auto increment fields in PostgreSQL tables
Posted: 30th December 2009 by Tim in PostgreSQLTags: auto increment. serial, field, PostgreSQL, table
Database systems such as MySQL allow you to specify a column as being auto incrementing. PostgreSQL, on the other hand, does things differently. Fields are incremented using sequences; external counters which are incremented manually. To use this requires setting up the sequence with a unique name and telling PostgreSQL to grab and increment the sequence [...]
SHOW FULL PROCESSLIST in PostgreSQL
Posted: 21st December 2009 by Tim in MySQL, PostgreSQLTags: activity, MySQL, PostgreSQL, processlist, stats
Like MySQL, there is a way to see all processes in PostgreSQL. It’s one simple command: SELECT * FROM pg_stat_activity; Note that if the current_query field keeps coming up empty, you might need to enable stats_command_string in your postgresql.conf. Enjoy.
PQexecParams example – PostgreSQL query execution with parameters
Posted: 19th November 2009 by Tim in C, PostgreSQLTags: C, coding, exec, parameter, PostgreSQL, programming, query
The PostgreSQL documentation states that PQexecParams can be called like so: PGresult *PQexecParams(PGconn *conn, const char *command, int nParams, const Oid *paramTypes, const char * const *paramValues, const int *paramLengths, const int *paramFormats, int resultFormat); There are two parts of this call which can be tricky to novice C and/or PostgreSQL users and aren’t explained [...]
Resetting SERIAL counters in PostgreSQL
Posted: 19th November 2009 by Tim in PostgreSQLTags: auto increment, clear, field, increment, PostgreSQL, primary key, reset, sequence, serial, table
There are times when you want to explicitly set or reset the counter value on a SERIAL (auto incrementing) field or on a SEQUENCE. This may be done using a SELECT SETVAL(…) command. If you’re using a SERIAL field, PostgreSQL will actually create a SEQUENCE for you, which increments every time you insert a row. [...]
USE [schema_name] in PostgreSQL
Posted: 17th November 2009 by Tim in PostgreSQLTags: database, MySQL, PostgreSQL, select, SET, USE
For those moving from MySQL to PostgreSQL, there are a few differences which you will need to get used to. One of these differences is the USE command, used to select the schema to select tables, views, etc from. PostgreSQL does not have the USE command. Instead, you can use: SET search_path TO [schema_name] For [...]