sqlite: More guidance on avoiding NULLs.

Change-Id: I60a262dc0a8d37bad44615bba98d5ff8970a0110
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2918006
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Reviewed-by: Ayu Ishii <ayui@chromium.org>
Commit-Queue: Victor Costan <pwnall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#886843}

sql/README.md

@@ -276,12 +276,21 @@ using `sql::Statement::ColumnTime()`.
 Column types should not include information ignored by SQLite, such as numeric
 precision or scale specifiers, or string length specifiers.
 
-Columns should have the `NOT NULL` constraint whenever possible. This saves
-maintainers from having to reason about the less intuitive cases of
-[`NULL` handling](https://sqlite.org/nulls.html).
-
-Columns should avoid `DEFAULT` values. This moves the burden of checking that
-`INSERT` statements aren't missing any columns from the code reviewer to SQLite.
+Columns should have
+[`NOT NULL` constraints](https://sqlite.org/lang_createtable.html#not_null_constraints)
+whenever possible. This saves maintainers from having to reason about the less
+intuitive cases of [`NULL` handling](https://sqlite.org/nulls.html).
+
+`NOT NULL` constraints must be explicitly stated in column definitions that
+include `PRIMARY KEY` specifiers. For historical reasons, SQLite
+[allows NULL primary keys](https://sqlite.org/lang_createtable.html#the_primary_key)
+in most cases.  When a table's primary key is composed of multiple columns,
+each column's definition should have a `NOT NULL` constraint.
+
+Columns should avoid `DEFAULT` values. Columns that have `NOT NULL` constraints
+and lack a `DEFAULT` value are easier to review and maintain, as SQLite takes
+over the burden of checking that `INSERT` statements aren't missing these
+columns.
 
 Surrogate primary keys should use the column type `INTEGER PRIMARY KEY`, to take
 advantage of SQLite's rowid optimizations.

third_party/sqlite/sqlite3_shim.c

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// This is a shim that injects Chrome-specific defitions into sqlite3.c
+// This is a shim that injects Chrome-specific definitions into sqlite3.c
 // BUILD.gn uses this instead of building the sqlite3 amalgamation directly.
 
 // We prefix chrome_ to SQLite's exported symbols, so that we don't clash with