How to create a temporary table, if it does not already exist, and add the selected rows to it?
CREATE TABLE AS
is the simplest and fastest way:
CREATE TEMP TABLE tbl AS
SELECT * FROM tbl WHERE ... ;
Do not use SELECT INTO. See:
Combine two tables into a new one so that select rows from the other one are ignored
Not sure whether table already exists
CREATE TABLE IF NOT EXISTS ... was introduced in version Postgres 9.1.
For older versions, use the function provided in this related answer:
PostgreSQL create table if not exists
Then:
INSERT INTO tbl (col1, col2, ...)
SELECT col1, col2, ...
Chances are, something is going wrong in your code if the temp table already exists. Make sure you don't duplicate data in the table or something. Or consider the following paragraph ...
Unique names
Temporary tables are only visible within your current session (not to be confused with transaction!). So the table name cannot conflict with other sessions. If you need unique names within your session, you could use dynamic SQL and utilize a SEQUENCE:
Create once:
CREATE SEQUENCE tablename_helper_seq;
You could use a DO statement (or a plpgsql function):
DO
$do$
BEGIN
EXECUTE
'CREATE TEMP TABLE tbl' || nextval('tablename_helper_seq'::regclass) || ' AS
SELECT * FROM tbl WHERE ... ';
RAISE NOTICE 'Temporary table created: "tbl%"' || ', lastval();
END
$do$;
lastval() and currval(regclass) are instrumental to return the dynamically created table name.
Related
I am struggling a bit on some dynamic postgresql :
I have a table named "list_columns" containing the columns names list with "column_name" as the variable name; those column names come from an input table called "input_table".
[list_columns]
column_name
col_name_a
col_name_b
col_name_c...
[input_table]
col_name_a
col_name_b
col_name_c
col_name_d
col_name_e
value_a_1
value_b_1
value_c_1
value_d_1
value_e_1
value_a_2
value_b_2
value_c_2
value_d_2
value_e_2
...
...
...
...
...
What I'd like to do is dynamically create a new table using that list, something like this:
create table output_table as
select (select distinct(column_name) seperated by "," from list_columns) from input_table;
The resulting table would be
[output_table]
col_name_a
col_name_b
col_name_c
value_a_1
value_b_1
value_c_1
value_a_2
value_b_2
value_c_2
...
...
...
I saw I should use some execute procedures but I can't figure out how to do so.
Note: I know i could directly select the 3 columns; I oversimplied the case.
If someone would be kind enough to help me on this,
Thank you,
Regards,
Jonathan
You need dynamic SQL for this, and for that you need PL/pgSQL.
You need to assemble the CREATE TABLE statement based on the input_table, then run that generated SQL.
do
$$
declare
l_columns text;
l_sql text;
begin
-- this generates the list of columns
select string_agg(distinct column_name, ',')
into l_columns
from list_table;
-- this generates the actual CREATE TABLE statement using the columns
-- from the previous step
l_sql := 'create table output_table as select '||l_columns||' from input_table';
-- this runs the generated SQL, thus creating the output table.
execute l_sql;
end;
$$;
If you need that a lot, you can put that into a stored function (your unsupported Postgres version doesn't support real procedures).
I need to insert either set A or set B of records into a #temptable, depending on certain condition
My pseudo-code:
IF OBJECT_ID('tempdb..#t1') IS NOT NULL DROP TABLE #t1;
IF {some-condition}
SELECT {columns}
INTO #t1
FROM {some-big-table}
WHERE {some-filter}
ELSE
SELECT {columns}
INTO #t1
FROM {some-other-big-table}
WHERE {some-other-filter}
The two SELECTs above are exclusive (guaranteed by the ELSE operator). However, SQL compiler tries to outsmart me and throws the following message:
There is already an object named '#t1' in the database.
My idea of "fixing" this is to create #t1 upfront and then executing a simple INSERT INTO (instead of SELECT... INTO). But I like minimalism and am wondering whether this can be achieved in an easier way i.e. without explicit CREATE TABLE #t1 upfront.
Btw why is it NOT giving me an error on a conditional DROP TABLE in the first line? Just wondering.
You can't have 2 temp tables with the same name in a single SQL batch. One of the MSDN article says "If more than one temporary table is created inside a single stored procedure or batch, they must have different names". You can have this logic with 2 different temp tables or table variable/temp table declared outside the IF-Else block.
Using a Dyamic sql we can handle this situation. As a developoer its not a good practice. Best to use table variable or temp table.
IF 1=2
BEGIN
EXEC ('SELECT 1 ID INTO #TEMP1
SELECT * FROM #TEMP1
')
END
ELSE
EXEC ('SELECT 2 ID INTO #TEMP1
SELECT * FROM #TEMP1
')
I know that to drop a table in a database is as follows, but if I have a database that has over a dozen tables and I only need to keep 3 of them, am I able to delete all but the 3 with only one command?
DROP TABLE IF EXISTS c_note RESTRICT;
Yes, but you need to enumerate over all the tables you want to drop. There's no command to drop all but 3. So, if you had the following tables:
foo
bar
baz
narf
poit
troz
And you wanted to drop the first three and keep the last three, you would use the command:
DROP TABLE foo, bar, baz;
If you know the tables all exist, there's no need for IF EXISTS, although it won't hurt. RESTRICT is also not needed -- that's the default (the opposite is CASCADE, where you also drop dependant objects).
SQL Doc
The following query will simmply delete all the tables but keep your schema
select 'drop table if exists ' || tablename || ' cascade;'
from pg_tables
where schemaname = 'public'
Write down a query where you can skip three tables by name
select 'drop table if exists ' || tablename || ' cascade;'
from pg_tables
where schemaname = 'public'
and tablename not in (:tbaleNamesToKeep);
I had 192 tables, so manually writing all tables out was infeasible. My strategy was to select all table names, remove the tables I wanted to keep and then run a DROP on the rest.
1. Select all tables
SELECT tablename FROM pg_tables
When you select, copy and paste the results, the text should look something like this:
"table1"
"table2"
"table3"
...
2. Remove the tables you want to keep
So let's say you want to keep table2, you just remove the line.
"table1"
"table3"
...
3. Add commas with text replacement
Replace "\n with ",\n.
To do this, I use regexr.com, but any text editor will do.
"table1",
"table3",
...
4. Create and run the final PSQL statement
DROP TABLE
"table1",
"table3",
...
I am using postgreSQL. I want to select data from a table. Such table name contains the current year. such as abc2013. I have tried
select * from concat('abc',date_part('year',current_date))
select *from from concat('abc', extract (year from current_date))
So how to fetch data from such table dynamically?
Please don't do this - look hard at alternatives first, starting with partitioning and constraint exclusion.
If you must use dynamic table names, do it at application level during query generation.
If all else fails you can use a PL/PgSQL procedure like:
CREATE OR REPLACE pleasedont(int year) RETURNS TABLE basetable AS $$
BEGIN
RETURN QUERY EXECUTE format('SELECT col1, col2, col3 FROM %I', 'basetable_'||year);
END;
$$ LANGUAGE plpgsql;
This will only work if you have a base table that has the same structure as the sub-tables. It's also really painful to work with when you start adding qualifiers (where clause constraints, etc), and it prevents any kind of plan caching or effective prepared statement use.
I want to use SELECT INTO to make a temporary table in one of my functions. SELECT INTO works in SQL but not PL/pgSQL.
This statement creates a table called mytable (If orig_table exists as a relation):
SELECT *
INTO TEMP TABLE mytable
FROM orig_table;
But put this function into PostgreSQL, and you get the error: ERROR: "temp" is not a known variable
CREATE OR REPLACE FUNCTION whatever()
RETURNS void AS $$
BEGIN
SELECT *
INTO TEMP TABLE mytable
FROM orig_table;
END; $$ LANGUAGE plpgsql;
I can SELECT INTO a variable of type record within PL/pgSQL, but then I have to define the structure when getting data out of that record. SELECT INTO is really simple - automatically creating a table of the same structure of the SELECT query. Does anyone have any explanation for why this doesn't work inside a function?
It seems like SELECT INTO works differently in PL/pgSQL, because you can select into the variables you've declared. I don't want to declare my temporary table structure, though. I wish it would just create the structure automatically like it does in SQL.
Try
CREATE TEMP TABLE mytable AS
SELECT *
FROM orig_table;
Per http://www.postgresql.org/docs/current/static/sql-selectinto.html
CREATE TABLE AS is functionally similar to SELECT INTO. CREATE TABLE AS is the recommended syntax, since this form of SELECT INTO is not available in ECPG or PL/pgSQL, because they interpret the INTO clause differently. Furthermore, CREATE TABLE AS offers a superset of the functionality provided by SELECT INTO.