.Dd December 19, 2018
.Dt SQLITE3SESSION_DIFF 3
.Os
.Sh NAME
.Nm sqlite3session_diff
.Nd Load The Difference Between Tables Into A Session
.Sh SYNOPSIS
.Ft int 
.Fo sqlite3session_diff
.Fa "sqlite3_session *pSession"
.Fa "const char *zFromDb"
.Fa "const char *zTbl"
.Fa "char **pzErrMsg "
.Fc
.Sh DESCRIPTION
If it is not already attached to the session object passed as the first
argument, this function attaches table zTbl in the same manner as the
sqlite3session_attach() function.
If zTbl does not exist, or if it does not have a primary key, this
function is a no-op (but does not return an error).
.Pp
Argument zFromDb must be the name of a database ("main", "temp" etc.)
attached to the same database handle as the session object that contains
a table compatible with the table attached to the session by this function.
A table is considered compatible if it: 
.Bl -bullet
.It
Has the same name, 
.It
Has the same set of columns declared in the same order, and 
.It
Has the same PRIMARY KEY definition.
.El
.Pp
If the tables are not compatible, SQLITE_SCHEMA is returned.
If the tables are compatible but do not have any PRIMARY KEY columns,
it is not an error but no changes are added to the session object.
As with other session APIs, tables without PRIMARY KEYs are simply
ignored.
.Pp
This function adds a set of changes to the session object that could
be used to update the table in database zFrom (call this the "from-table")
so that its content is the same as the table attached to the session
object (call this the "to-table").
Specifically: 
.Bl -bullet
.It
For each row (primary key) that exists in the to-table but not in the
from-table, an INSERT record is added to the session object.
.It
For each row (primary key) that exists in the to-table but not in the
from-table, a DELETE record is added to the session object.
.It
For each row (primary key) that exists in both tables, but features
different non-PK values in each, an UPDATE record is added to the session.
.El
.Pp
To clarify, if this function is called and then a changeset constructed
using sqlite3session_changeset(), then after
applying that changeset to database zFrom the contents of the two compatible
tables would be identical.
.Pp
It an error if database zFrom does not exist or does not contain the
required compatible table.
.Pp
If the operation successful, SQLITE_OK is returned.
Otherwise, an SQLite error code.
In this case, if argument pzErrMsg is not NULL, *pzErrMsg may be set
to point to a buffer containing an English language error message.
It is the responsibility of the caller to free this buffer using sqlite3_free().
.Sh SEE ALSO
.Xr sqlite3session_attach 3 ,
.Xr sqlite3session_changeset 3