How to INSERT INTO table from dynamic query?

问题

My version of Postgres is:

"PostgreSQL 9.4.4, compiled by Visual C++ build 1800, 32-bit"

Let's say I have two tables Table1 and Table2, which are having column col1 and col2 respectively.

CREATE TABLE Table1(col1 int);
CREATE TABLE Table2(col2 int);

There is another table Table3 storing a formula for migrating data from Table1 to Table2:

CREATE TABLE Table3 (     
  tbl_src   varchar(200),
  col_src   varchar(500),
  tbl_des   varchar(200),
  col_des   varchar(100),
  condition varchar(500)
);

INSERT INTO Table3 (tbl_src, col_src, tbl_des, col_des, condition)
SELECT 'Table1','col1','Table2','col2',NULL

How to compile this formula in a dynamic query and insert into the destination table?

回答1:

The basic query to build the command dynamically:

SELECT format('INSERT INTO %I (%I) SELECT %I FROM %I'
            , tbl_des, col_des, col_src, tbl_src) As sql
FROM   table3;

This produces a query like:

INSERT INTO "Table2" (col2) SELECT col1 FROM "Table1"

Note the quoted upper-case spelling. Unlike in SQL commands, where unquoted identifiers are folded to lower-case automatically, the strings in your table are now case-sensitive!

Are PostgreSQL column names case-sensitive?

I suggest you never double-quote identifiers and use legal, lower-case names exclusively.

To automate:

DO
$$BEGIN
   EXECUTE (
      SELECT format('INSERT INTO %I (%I) SELECT %I FROM %I'
                  , tbl_des, col_des, col_src, tbl_src) As sql
      FROM   table3
      -- WHERE table3_id = 123  -- select only *one* row!
      );
END$$;

You need to understand the format() function. Read the manual.

You can wrap this into a plpgsql function as well and pass additional parameters: