[nosoop]

Here's my current approach to achievements; I wrote my own personal plugins for it.
SQLite, but should be easy to adapt to SQL:

/* run with sqlite3 -init "configs/sql-init-scripts/sqlite/custom_achievements.sql" "data/sqlite3/custom-achievements.sq3" */

/* list of achievements */
CREATE TABLE achievements (
    achievement_id INTEGER PRIMARY KEY AUTOINCREMENT,
    name TEXT NOT NULL,
    displaytype INTEGER, /* now deprecated */
    visible BOOLEAN DEFAULT TRUE, /* determines whether or not it's rendered on an achievement page */
    CONSTRAINT achievement_name UNIQUE(name)
);

/* per-account achievement status */
CREATE TABLE achievement_status (
    achievement_id INTEGER,
    steamid3 INTEGER NOT NULL,
    achieved INTEGER DEFAULT 0, /* should be time achieved */
    metadata TEXT,
    CONSTRAINT achievement_user UNIQUE(achievement_id, steamid3)
);

/* localized achievement name and descriptions for display */
/* also to be used in CallOnCustomAchievementAwarded() to print to chat */
CREATE TABLE achievement_languages (
    achievement_name TEXT NOT NULL,
    language_shortcode TEXT NOT NULL,
    achievement_local_name TEXT NOT NULL,
    achievement_local_description TEXT,
    CONSTRAINT achievement_in_language UNIQUE(achievement_name, language_shortcode)
);

/* automatically create language placeholder entries */
CREATE TRIGGER create_achievement_localizations AFTER INSERT ON achievements BEGIN
    INSERT INTO achievement_languages (achievement_name, language_shortcode,
            achievement_local_name, achievement_local_description) VALUES
            (NEW.name, 'en', NEW.name, NEW.name || '_description');  
END;

One core plugin handles all the database stuff; other plugins "register" achievements by unique name (getting the achievement_id back as its this methodmap value) and use the core plugin natives to grant / update achievement data. It's kind of like client preferences / cookies, with some additional columns.

• The methodmap has the following natives: register (return the ID of an achievement based on the input name, inserting a row for it if it doesn't exist), award to account, fetch / set info, and reset.
• Unlocking an achievement records the time it was unlocked (so you can have unlocked date / time). You can "re-lock" achievements by setting the time to 0 (so it's a "falsy" value), and wiping the metadata.
• Metadata is additional storage for the account / achievement to track long-term achievements. It's up to any plugins / external applications that care to determine how it's parsed, allowing for more complex displays than what Steam offers. Fetching the metadata is done through a threaded query.
• Localization is input manually and separately, other than providing placeholders for your language(s) via the AFTER INSERT trigger.
• Localization data is fetched into a KeyValues handle with language shortcodes as sections; achievement announcements can be done by getting each client's language, going into the section (falling back to the server's by default), and pulling the appropriate name.

For items, I'd imagine I'd do something based off of Valve's inventory approach (note: I don't know the internals of Valve's database): I'd use a table for player items (unique IDs per item / inventory space), a table for custom attribute overrides (names, descriptions, skins, etc. based on item unique ID), maybe a table for the schema too.