PostgreSQL create table if not exists

问题:

In a MySQL script you can write:

CREATE TABLE IF NOT EXISTS foo ...; 

... other stuff ...

and then you can run the script many times without re-creating the table.

How do you do this in PostgreSQL?

回答1:

This feature has been implemented in Postgres 9.1:

CREATE TABLE IF NOT EXISTS myschema.mytable (i integer); 

---

For older versions, here is a function to work around it:

CREATE OR REPLACE FUNCTION create_mytable ()   RETURNS void AS $func$ BEGIN    IF EXISTS (SELECT 1 FROM pg_catalog.pg_tables                WHERE  schemaname = 'myschema'               AND    tablename  = 'mytable') THEN       RAISE NOTICE 'Table "myschema"."mytable" already exists.';    ELSE       CREATE TABLE myschema.mytable (i integer);    END IF; END $func$ LANGUAGE plpgsql; 

Call:

SELECT create_mytable();        -- call as many times as you want.  

If the user does not have the necessary privileges to create the table you might want to use SECURITY DEFINER. This version is safe enough.

回答2:

Try this :

CREATE TABLE IF NOT EXISTS app_user (    username varchar(45) NOT NULL,    password varchar(450) NOT NULL,     enabled integer NOT NULL DEFAULT '1',     PRIMARY KEY (user_id)   ) 

回答3:

I created a generic solution out of the existing answers which can be reused for any table:

CREATE OR REPLACE FUNCTION create_if_not_exists (table_name text, create_stmt text) RETURNS text AS $_$ BEGIN  IF EXISTS (     SELECT *     FROM   pg_catalog.pg_tables      WHERE    tablename  = table_name     ) THEN    RETURN 'TABLE ' || '''' || table_name || '''' || ' ALREADY EXISTS'; ELSE    EXECUTE create_stmt;    RETURN 'CREATED'; END IF;  END; $_$ LANGUAGE plpgsql; 

Usage:

select create_if_not_exists('my_table', 'CREATE TABLE my_table (id integer NOT NULL);'); 

It could be simplified further to take just one parameter if one would extract the table name out of the query parameter. Also I left out the schemas. Feel free to extend my solution if you know how to do that - I'm not that deep into plpgsql yet (this is the first time I'm dealing with it).

回答4:

There is no CREATE TABLE IF NOT EXISTS... but you can write a simple procedure for that, something like:

CREATE OR REPLACE FUNCTION execute(TEXT) RETURNS VOID AS $$ BEGIN   EXECUTE $1; END; $$ LANGUAGE plpgsql;   SELECT    execute($$       CREATE TABLE sch.foo        (         i integer       )   $$)  WHERE    NOT exists    (     SELECT *      FROM information_schema.tables      WHERE table_name = 'foo'       AND table_schema = 'sch'   ); 

That's a little bit weird, but can simply be

回答5:

This solution is somewhat similar to the answer by Erwin Brandstetter, but uses only the sql language.

Not all PostgreSQL installations has the plpqsql language by default, this means you may have to call CREATE LANGUAGE plpgsql before creating the function, and afterwards have to remove the language again, to leave the database in the same state as it was before (but only if the database did not have the plpgsql language to begin with). See how the complexity grows?

Adding the plpgsql may not be issue if you are running your script locally, however, if the script is used to set up schema at a customer it may not be desirable to leave changes like this in the customers database.

This solution is inspired by a post by Andreas Scherbaum.

-- Function which creates table CREATE OR REPLACE FUNCTION create_table () RETURNS TEXT AS $$     CREATE TABLE table_name (        i int     );     SELECT 'extended_recycle_bin created'::TEXT;     $$ LANGUAGE 'sql';  -- Test if table exists, and if not create it SELECT CASE WHEN (SELECT true::BOOLEAN     FROM   pg_catalog.pg_tables      WHERE  schemaname = 'public'     AND    tablename  = 'table_name'   ) THEN (SELECT 'success'::TEXT)   ELSE (SELECT create_table()) END;  -- Drop function DROP FUNCTION create_table(); 

回答6:

There is no CREATE TABLE IF NOT EXISTS... but you can write a simple procedure for that, something like:

CREATE OR REPLACE FUNCTION prc_create_sch_foo_table() RETURNS VOID AS $$ BEGIN  EXECUTE 'CREATE TABLE /* IF NOT EXISTS add for PostgreSQL 9.1+ */ sch.foo (                     id serial NOT NULL,                      demo_column varchar NOT NULL,                      demo_column2 varchar NOT NULL,                     CONSTRAINT pk_sch_foo PRIMARY KEY (id));                    CREATE INDEX /* IF NOT EXISTS add for PostgreSQL 9.5+ */ idx_sch_foo_demo_column ON sch.foo(demo_column);                    CREATE INDEX /* IF NOT EXISTS add for PostgreSQL 9.5+ */ idx_sch_foo_demo_column2 ON sch.foo(demo_column2);'                WHERE NOT EXISTS(SELECT * FROM information_schema.tables                          WHERE table_schema = 'sch'                              AND table_name = 'foo');           EXCEPTION WHEN null_value_not_allowed THEN            WHEN duplicate_table THEN            WHEN others THEN RAISE EXCEPTION '% %', SQLSTATE, SQLERRM;  END; $$ LANGUAGE plpgsql; 

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