Answers for Salesforce Data Geeks

10 Salesforce Best Data Migration Tools 2015Review of Data Loaders. More cloud-oriented than Top 10 ETL Tools for Salesforce Data Migration below
15-Char IDs to 18-Char IDs How to create a formula field that converts 15-character IDs to 18-character IDs.
About Archived Activities How Salesforce handles the archiving of activities, and where activities can be found once they're archived.
About Audit Fields How Salesforce handles system "Audit Fields," and how you can write values to them.  The editable Audit Fields are:
• Created By
• Created Date
• Last Modified By
• Last Modified Date

To write those values, the "Create Audit Fields" permission must be (1) activated on your org, then (2) on a Profile or Permission Set, before you can write values to these fields:

1) Setup|
Customize|User Interface
  • Enable "Set Audit Fields upon Record Creation" and "Update Records with Inactive Owners" User Permissions
2) Setup|Manage Users|Profiles|[ProfileName]
    or
    Setup|Manage Users|Permission Sets|[PermSet]
  • Set Audit Fields upon Record Creation
  • Update Records with Inactive Owners
Click here for more information about Audit Fields.
Add Workflow Email Alerts to Activity History Workflow Email Alerts are not normally added to a record's Activity History. Here's a workaround. (Also feel free to vote for this idea.)
Administrative Pre-Work for Any Major Salesforce Development Project From Andrew Sinclair's blog:

As a Salesforce consultant I like to add value to an orginization through their use of Salesforce, but that quickly gets complicated when my first recommendation is to cleanup Salesforce, before any significant feature development can take place.

It’s always a challenge when the client wants new features to drive efficiencies and increase adoption, but they’re stuck in a situation where they need to crawl out of debt first. My recommendations to clean up Salesforce – and keep it clean – are the following.
Adoption Best Practices A collection of presentations and other docs from salesforce.com about how to get users using Salesforce. For my two cents, it all boils down to Executive Sponsorship and a good Change Management plan!
Application Limits • App Setup Limits
• Custom Field Limits
• Rules Limits
• Picklist Limits
• Custom Picklist Limits
• Concurrent API Limits
• Total API Limits
• Sites Limits
• Storage Limits
Assigning Records to Inactive Owners It is possible to assign records to inactive Owners. (Yah, I know, I couldn't believe it at first, either.)
Autocreating a Contact from Web-To-Case or Email-To-Case The best way to auto-create Contacts from Cases is to use an Apex trigger. Blog post contains trigger and test code.
Calculating Day of the Week in a Formula Field Have you ever had a need to know the day of the week an opportunity closed? What about excluding Saturdays and Sundays from your reports so that your dashboards only show business days? Day of week calculations are easy with formula fields, using the technique described.

This example calculates the day of the week as a number (0 = Saturday, 1 = Sunday, 2 = Monday, 3 = Tuesday, etc.) for Opportunity Close Date.
Chatter File ThumbnailsHere's the code for creating & viewing thumbnails of Chatter Files. There's an app here.
Collect and update user data during the login processOMG, this is my new favorite thing. Salesforce's new Login Flows feature makes it possible to fire a Trigger (okay, fire a Flow that fires Apex) when a user logs in. woohoo!
Comparing Import Wizards There are actually two distinct Import Wizards for Accounts & Contacts, with somewhat different capabilities. Do you know how they differ?
Consultant Resource Center Useful links for consultants, published by salesforce.com--I especially like that there're dedicated pages for Data Migration and Large Data Volumes
Content Search File Types When I run a Search in Content, what files does it search? What are the maximum sizes of the files searched?
Converting Leads Help topic on the ins & outs of what happens when a Lead is converted.
Creating a Many-to-Many Relationship How do I create a many-to-many relationship (aka a "junction object")?
Cross-Object Workflow Field Updates The specifics dictating when a Workflow Field Update can write values to other objects' fields. Subtle, but very handy to know!
Currencies: Inserting and UpdatingHandy insights for loading multi-currency data in orgs (eg, "what happens when I change a record's CurrencyIsoCode?").
Currencies: Querying and FilteringHandy insights for pulling multi-currency data from orgs (eg, "how do I filter multi-currency records on a single currency amount?"). 
Data Best Practices A collection from salesforce.com of Dreamforce presentations and random useful links.
Data Demystified Links and tips for advanced Salesforce data users; covers Large Data Volumes, handling query timeouts, and links to power tools, among other goodies.
Data Loader ComparisonShowdown of the free data loaders!
Jitterbit Cloud Data Loader vs Dataloader.io vs Informatica Data Wizard
Date Formats and Date Literals The how-to for formatting Date and DateTime values in ways that make the API happy.
Deciding If Lightning Is Right for Your CompanyLightning Experience (LE) versus Salesforce Classic: which UI does what?

(Appirio made a decision flowchart that is imo a little too bullish on LE.)
Developer Cheat Sheets PDFs with help for Apex, formulas, Visualforce, and the Web Services API. Recently added: Chatter Cheat Sheet
Developer Tools Salesforce.com's master page of all the various geek tools.
Development Lifecycle Guide Salesforce.com's doc for managing an org: managing releases, running tests, moving changes between environments.
Dynamically-Parameterized Reports Create a report that takes its criteria from the page you're currently on (eg, "show me all Accounts in the same region as this Account").
Edition Comparison A summary of what each version of Salesforce offers.
Enforcing Unique Records
One way to guarantee unique records on, say, a quota table.
Entity Relationship Diagram (ERD) Can't spell NERD without it!  These diagrams show how standard Salesforce objects connect to one another.  To create one from scratch, I recommend installing ERD Tool.
Exporting from ACT/Goldmine/ Outlook/etc How to export to CSV from various contact apps (requires Salesforce login).
File Size Limits The upload size limits for Attachments, Chatter Attachments, Content, Documents, Email Attachments, and Google Docs.
Force.com Commandments Jonathan Rico's useful list of design principles when creating Force.com apps.
Formula Reference A few goodies for creating formulas:
Getting Started with Image Fields Display flags, red/yellow/green lights, star ratings, and more--all inline on standard Salesforce pages. This old-school PDF walks you through it.
"How Do I...?" Videos Sales Engineer Alex Zarowny has posted over 30 short videos walking through administrator procedures like creating a simple Workflow Rule, creating a Report, and renaming Tabs.
How to Create a Report of Records Owned by Inactive Users This is a very popular request for customers who want to know which accounts, leads, opportunities are still owned by users who are inactive. That way they can reassign them to active sales reps.
How to Upload Content Using Data Loader The simplest procedure I've yet discovered for mass-uploading Content.
How to Use Connect for Office with Sandbox Connect for Office, which enables (1) embedding reports in Excel spreadsheets and (2) customizing Mail Merge templates in Word, can also work with Sandbox data. Here's how.
Importing Attachments Using Data Loader Help topic for importing Attachments into Salesforce. Same basic procedure works for Documents, too.
Importing Non-English Characters into Salesforce Help topic for importing foreign characters (typically Asian characters) into Salesforce.
Importing Products Help topic for importing Products into Salesforce. Useful for navigating the tricky shoals of Pricebooks, Products, PricebookEntries, and the rest of it. 
Inline Account Hierarchy Cool AppExchange offering that makes it possible to show the entire Account Hierarchy right on the Account Detail page, like this:
Mass Import Customer Portal Users How to create Customer Portal Users through the API.
Modify All Data Salesforce.com's documentation of what's affected by activating the "Modify All Data" permission.
Migrating Converted Leads Tips for migrating already-converted Leads into Salesforce. Thanks to TCS's Ajay Kumar for writing the topic!
Migrating Field History Two okayish workarounds for migrating Salesforce field history.
Minimize Apex Test Failures I don't touch on much Apex stuff here, but this one is too good to pass up: create a custom testing object to minimize Apex test failures for your Appexchange customers.
Most-Recently-Won Opportunity Date & Amount A cool example of the advanced use of formula fields. Shows each Account's most-recently-won Opportunity Date and Amount.
MS Excel Tools for SalesforceReviews of Excel Connector, Salesforce Reports for Excel, Power Query, Bulk Object Field Creator, Enabler4Excel, and X-Author.
Order of Execution Validation Rules, Triggers, Assignment Rules, Workflow Rules, Auto-Response Rules, Escalation Rules, Rollup Summary Fields--when you save a record with an insert, update, or upsert statement, what fires? And in what order?
Override Default Tabs I don't want my users to see "Recent Accounts" when they click on the "Accounts" tab. Is there a simple way to fix this?
Owner Info in Formulas Workaround While you can't use cross-object formulas on the Owner of a record... you CAN use a custom lookup relationship to the User object. So we're going to create our own lookup field and the use a trigger to make sure it is always synchronized with the standard Owner Field.
Partner Types How do Gold Partners differ from Silver Partners? Thanks to Shell Black for this handy table (which strangely isn't in Salesforce Help).
Quick Case Close Button People often ask me how they can get around the Close Case page. One option to do this is to replace the standard Close Case button with a custom button of your own. It's remarkably easy to do! Here's how.
Quick Email Button Sending an email from Salesforce.com can be a clicky operation. You can make this a much less clicky operation by making a custom button which changes the URL parameters.

retURL = Item/Object to return to after email is sent
p2_lkid = To address
p3_lkid = Related object
p4= CC address
p5 = BCC address
p24=Additional To
p26 = From address
template_id = Email template ID

If you need the email to send itself, read this.
Rollup Summary Fields for Lookup Relationships How do I create a Rollup Summary Field (RSF) for two objects that are connected by a lookup relationship? Jeff Douglas's blog has the answer.
Salesforce Disabling TLS 1.0When and how Salesforce is disabling TLS 1.0 for Sandbox and Production orgs, and how that affects data and data tools.
Salesforce Quick Tips A selection of quick tips, time-savers, and tricks you can add to your Salesforce implementation today. Only four videos at the moment, but they're pretty good.
Search Fields "Which objects' fields are searchable?" Authoritative object-by-object list that methodically lays out why Global Search is so cool.
Siteforce Examples "What kinds of sites can I create with Siteforce?" Examples and links are listed on this salesforce.com page.
SOQL Tips This article, "A Deeper look at SOQL and Relationship Queries on Force.com", provides cool examples of advanced SOQL queries.
Standard Object ID Prefixes A mighty damn comprehensive list of Salesforce objects' three-character prefixes (you did know that the first three characters of an ID tell you what object you're looking at, right?). From Salesforce developer site FishOfPrey.
The Salesforce Bulk API - Maximizing Parallelism...Handy reference for understanding how to best use the Bulk API when loading data. It really helped me understand why I kept getting record-lock errors.
Time-Based Workflow FAQ If, like me, you're constantly forgetting what happens when time-based Workflow Rules meet record updates/deletions, this link is for you.
Top 10 ETL Tools for Salesforce Data MigrationRundown of ETL tools, starting with Data Loader, Dataloader.io, and Jitterbit Cloud Data Loader--and seven others. Some are not free.
Top 15 Free Extract, Transform, and Load (ETL) SoftwareDescriptions of 15 free ETL packages. I've only started going through these myself; I've tried Talend, which appears really powerful, but has been hard to set up.
User License Comparison Which User Types can do what?
Using Data Loader from the Command Line How can I run Data Loader programmatically? How can I write Data Loader scripts? Through the Command Line! I used this article to write a lot of the code behind Synchronizer. Very handy.

For a higher-level explanation, use this.
Using External ID Fields in Salesforce What the heck are External IDs? Why would I want to use them? And what is "Upsert"? Someday (in my Copious Free Time) I'll write a true step-by-step for using Data Loader's Upsert command. In the meantime, this blog entry from Jeff Douglas is a good introduction to the basics.
Using Talend to Export Data from Salesforce.com How to get up & running with the free integration tool Talend Open Studio. More posts about Talend+Salesforce from Appirio's Tech Blog available here and here.
Valid CSV Record Rows The Bulk API (which some apps confusingly call "batch mode") uses a strict format for field values to optimize processing for large sets of data. Note the following when generating CSV files that contain Salesforce.com records.
Viewing Non-English CSV Characters in Excel Help topic for viewing a CSV's foreign characters (typically Asian characters) in Excel.
Which Objects Count Against My Storage Limit? Storage is divided into two categories: data storage and file storage. File storage includes files in attachments, the Documents tab, the File field, and Salesforce CRM Content. Data storage includes the objects listed...
Why Use Standard Address Fields? Before suppressing the standard address fields and creating a custom set of address fields, consider the caveats listed in this help topic.
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