MacLochlainns Weblog

This blog post addresses how to convert a list of values into a structure (in C/C++ its a struct, in Java its an ArrayList, and PL/pgSQL it’s an array of a type). The cast_strings function converts a list of strings into a record data structure. It calls the verify_date function to identify a DATE data type and uses regular expressions to identify numbers and strings.

You need to build the struct type below first.

CREATE TYPE struct AS
( xnumber  DECIMAL
, xdate    DATE
, xstring  VARCHAR(100));

The cast_strings function is defined below:

CREATE FUNCTION cast_strings
( pv_list  VARCHAR(10)[] ) RETURNS struct AS
  $$
  DECLARE
  /* Declare a UDT and initialize an empty struct variable. */
  lv_retval  STRUCT := (null, null, null); 
  BEGIN  
    /* Loop through list of values to find only the numbers. */
    FOR i IN 1..ARRAY_LENGTH(pv_list,1) LOOP
      /* Order if statements by evaluation. */
      CASE
        /* Check for a value with only digits. */
        WHEN lv_retval.xnumber IS NULL AND REGEXP_MATCH(pv_list[i],'^[0-9]+$') IS NOT NULL THEN
          lv_retval.xnumber := pv_list[i];
        /* Check for a valid date. */
        WHEN lv_retval.xdate IS NULL AND verify_date(pv_list[i]) IS NOT NULL THEN
          lv_retval.xdate := pv_list[i];
        /* Check for a string with characters, whitespace, and digits. */
        WHEN lv_retval.xstring IS NULL AND REGEXP_MATCH(pv_list[i],'^[A-Za-z 0-9]+$') IS NOT NULL THEN
          lv_retval.xstring := pv_list[i];
        ELSE
          NULL;
      END CASE;
    END LOOP;
 
    /* Print the results. */
    RETURN lv_retval;
  END;
$$ LANGUAGE plpgsql;

There are two test cases for the cast_strings function. One uses a DO-block and the other a query.

• The first use-case checks with a DO-block:

  DO $$ DECLARE lv_list VARCHAR(11)[] := ARRAY['86','1944-04-25','Happy']; lv_struct STRUCT; BEGIN /* Pass the array of strings and return a record type. */ lv_struct := cast_strings(lv_list);   /* Print the elements returned. */ RAISE NOTICE '[%]', lv_struct.xnumber; RAISE NOTICE '[%]', lv_struct.xdate; RAISE NOTICE '[%]', lv_struct.xstring; END; $$;

  DO $$ DECLARE lv_list VARCHAR(11)[] := ARRAY['86','1944-04-25','Happy']; lv_struct STRUCT; BEGIN /* Pass the array of strings and return a record type. */ lv_struct := cast_strings(lv_list); /* Print the elements returned. */ RAISE NOTICE '[%]', lv_struct.xnumber; RAISE NOTICE '[%]', lv_struct.xdate; RAISE NOTICE '[%]', lv_struct.xstring; END; $$;

  It should return:

  psql:verify_pg.SQL:263: NOTICE: [86] psql:verify_pg.SQL:263: NOTICE: [1944-04-25] psql:verify_pg.SQL:263: NOTICE: [Happy]

  psql:verify_pg.sql:263: NOTICE: [86] psql:verify_pg.sql:263: NOTICE: [1944-04-25] psql:verify_pg.sql:263: NOTICE: [Happy]

  The program returns a structure with values converted into their appropriate data type.

• The second use-case checks with a query:

  WITH get_struct AS (SELECT cast_strings(ARRAY['99','2015-06-14','Agent 99']) AS mystruct) SELECT (mystruct).xnumber , (mystruct).xdate , (mystruct).xstring FROM get_struct;

  WITH get_struct AS (SELECT cast_strings(ARRAY['99','2015-06-14','Agent 99']) AS mystruct) SELECT (mystruct).xnumber , (mystruct).xdate , (mystruct).xstring FROM get_struct;

  It should return:

  xnumber | xdate | xstring ---------+------------+---------- 99 | 2015-06-14 | Agent 99 (1 row)

  xnumber | xdate | xstring ---------+------------+---------- 99 | 2015-06-14 | Agent 99 (1 row)

  The query defines a call to the cast_strings function with a valid set of values and then displays the elements of the returned structure.

As always, I hope this helps those looking for how to solve this type of problem. Just a quick reminder that this was written and tested in PostgreSQL 14.