Update wp_postmeta and wp_posts database tables with posts from another site

update.sql

# What if you are trying to migrate a bunch of posts from one site into
# another site that already has content? We'll assume you already  have
# a database export of the posts and postmeta you want to migrate. You
# will want to create a database locally to alter the tables to get them
# ready to merge into the production database that already has content.
# You will also need to know the highest `meta_id` and `ID` values from
# the `wp_postmeta` and `wp_posts` tables, respectively.

# First, we need to create new columns in the tables we have locally. To
# do this the right way, we are going to figure out what type of data
# needs to go into these columns (assuming you didn't already know off the
# top of your head.

show fields from `wp_postmeta`;

# This will show you all the columns in your selected database. In this 
# case, we'll be working with bigint(20) fields, so we need to create a
# new column that accepts the same type of data.

ALTER TABLE `wp_postmeta` ADD COLUMN id bigint(20) NOT NULL FIRST;

# Now we need to set the starting value to the highest value of `meta_id`
# on the destination database +1 and update all the values of our new
# column accordingly. 
# In this case, my last `meta_id` was 103546 so I'm starting with 103547.

set @index = 103547;
update `wp_postmeta` set id = (@index:=@index+1);

# Now we're ready to rename columns. We're going to start by renaming the
# `meta_id` column to `meta_id_old` and then we're going to rename `id`
# to `meta_id`.

ALTER TABLE wp_postmeta CHANGE meta_id meta_id_old bigint(20);
ALTER TABLE wp_postmeta CHANGE id meta_id bigint(20);

# Now we need to  make the new `meta_id` the primary key.

alter table wp_postmeta modify meta_id_old bigint, drop primary key, add primary key (meta_id);

# At this point you should be good to go, but if you didn't want to keep
# the old `meta_id_old` column around, you can drop that. Make a backup
# of your `wp_postmeta` table before you do this.

alter table wp_postmeta drop meta_id_old;

# Now we're going to do the same thing with `wp_posts`.
# Show the fields. We will learn that `ID` is also a bigint(20).
show fields from wp_posts;

# Create a new column.

alter table `wp_posts` add column newID bigint(20) not null first;

# My highest post ID is 8147, so setting my index at 8148.

set @index = 8148;
update `wp_posts` set newID = (@index:=@index+1);

# Rename tables.

ALTER TABLE wp_posts CHANGE ID old_ID bigint(20);
ALTER TABLE wp_posts CHANGE newID ID bigint(20);

# Drop the primary key off old_ID and set it to ID.

alter table wp_posts modify old_ID bigint, drop primary key, add primary key (ID);

# Backup your table here and drop the `old_ID` column.

alter table wp_posts drop old_ID;

References:
http://stackoverflow.com/questions/7989342/how-do-i-update-a-table-to-add-a-primary-key-and-update-all-of-the-existing-rows
http://stackoverflow.com/questions/9702766/sql-set-existing-column-as-primary-key-in-mysql
http://www.tech-recipes.com/rx/378/add-a-column-to-an-existing-mysql-table/
http://dba.stackexchange.com/questions/71353/how-to-change-mysql-table-auto-increment-primary-key