ATTACH DATABASE 'hash2.db' AS ha_2;
ATTACH DATABASE 'hash3.db' AS ha_3;

SELECT name, SUM(value) FROM (
  SELECT * FROM main.kv_store UNION ALL
  SELECT * FROM ha_2.kv_store UNION ALL
  SELECT * FROM ha_3.kv_store
) GROUP BY name ORDER BY SUM(value) DESC, name;

DETACH DATABASE ha_2;
DETACH DATABASE ha_3;

##</code><code>##

$ bash llil_sql_imp2 hash1.db big1.txt
$ bash llil_sql_imp2 hash2.db big3.txt
$ bash llil_sql_imp2 hash3.db big3.txt

$ time bash llil_sql_qryn hash1.db hash2.db hash3.db >out.txt

real  0m9.650s
user  0m9.534s
sys   0m0.106s

##</code><code>##

#!/usr/bin/env bash
# https://perlmonks.org/?node_id=11150248

# SQLite llil query multiple DBs.
# Usage: bash llil_sql_qryn hash1.db [ hash2.db ... ] >out.txt

# SQL

function make_script_init () {

  read -r -d '' var << 'EOF'
-- Set SQLite pragmas.
--
.output /dev/null
PRAGMA cache_size   = 10000;
PRAGMA page_size    = 4096;
PRAGMA journal_mode = OFF;
PRAGMA synchronous  = OFF;
PRAGMA temp_store   = MEMORY;
.output

-- Output records.
--
.mode csv
.headers off
.separator ROW "\n"
.separator "\t"
EOF

  echo "$var"
}

function make_script_query_one_db () {

  read -r -d '' var << 'EOF'
SELECT "name", SUM("value") FROM "kv_store"
  GROUP BY "name"
  ORDER BY SUM("value") DESC, "name";
EOF

  echo "$var"
}

function make_script_query_many_dbs () {

  ##
  # ATTACH DATABASE 'hash2.db' AS ha_2;
  # ATTACH DATABASE 'hash3.db' AS ha_3;
  # ATTACH DATABASE 'hashN.db' AS ha_N;
  #
  # SELECT name, SUM(value) FROM (
  #   SELECT * FROM main.kv_store UNION ALL
  #   SELECT * FROM ha_2.kv_store UNION ALL
  #   SELECT * FROM ha_3.kv_store UNION ALL
  #   SELECT * FROM ha_N.kv_store
  # ) GROUP BY name ORDER BY SUM(value) DESC, name;
  #
  # detach DATABASE ha_2;
  # detach DATABASE ha_3;
  # detach DATABASE ha_N;
  ##

  n=1 var=""

  for dbfile in "$@"; do
    n=$((n + 1))
    var+="ATTACH DATABASE '${dbfile}' AS ha_${n};${NL}"
  done

  var+="${NL}"
  var+="SELECT name, SUM(value) FROM (${NL}"
  var+="  SELECT * FROM main.kv_store UNION ALL${NL}"

  n=1

  for dbfile in "$@"; do
    n=$((n + 1))
    if [[ "$n" -le "$#" ]]; then
      var+="  SELECT * FROM ha_${n}.kv_store UNION ALL${NL}"
    else
      var+="  SELECT * FROM ha_${n}.kv_store${NL}"
    fi
  done

  var+=") GROUP BY name ORDER BY SUM(value) DESC, name;${NL}"
  var+="${NL}"

  n=1

  for dbfile in "$@"; do
    n=$((n + 1))
    var+="DETACH DATABASE ha_${n};${NL}"
  done

  echo "$var"
}

# RUN

DB="$1"; shift

if [[ ! "$DB" =~ .db$ ]]; then
  echo "usage: bash $0 hash.db [ hash2.db ... ]" >&2
  exit 1
elif [[ ! -r "$DB" ]]; then
  echo "$DB: cannot open '$DB' (No such file)" >&2
  exit 1
fi

SQLCMND=$( command -v sqlite3 )

if [[ -z "$SQLCMND" ]]; then
  echo "sqlite3: command not found" >&2
  exit 1
fi

NL=$'\n'

SQLSCRIPT="$( make_script_init )"
SQLSCRIPT+="${NL}${NL}"

if [[ "$#" -eq 0 ]]; then
  SQLSCRIPT+="$( make_script_query_one_db )"
elif [[ "$#" -gt 64 ]]; then
  echo "$DB: cannot attach more than 64 DBs" >&2
  exit 1
else
  for dbfile in "$@"; do
    if [[ ! -r "$dbfile" ]]; then
      echo "$dbfile: cannot open '$dbfile', exiting..." >&2
      exit 1
    fi
  done
  SQLSCRIPT+="$( make_script_query_many_dbs "$@" )"
fi

exec "$SQLCMND" "$DB" < <(echo "$SQLSCRIPT")