Find SQLite Column Names in Empty Table - database

Question

For kicks I'm writing a "schema documentation" tool that generates a description of the tables and relationships in a database. I'm currently shimming it to work with SQLite.
I've managed to extract the names of all the tables in a SQLite database via a query on the sqlite_master table. For each table name, I then fire off a simple
select * from <table name>
query, then use the sqlite3_column_count() and sqlite3_column_name() APIs to collect the column names, which I further feed to sqlite3_table_column_metadata() to get additional info. Simple enough, right?
The problem is that it only works for tables that are not empty. That is, the sqlite_column_*() APIs are only valid if sqlite_step() has returned SQLITE_ROW, which is not the case for empty tables.
So the question is, how can I discover column names for empty tables? Or, more generally, is there a better way to get this type of schema info in SQLite?
I feel like there must be another hidden sqlite_xxx table lurking somewhere containing this info, but so far have not been able to find it.

Answer 1

sqlite> .header on
sqlite> .mode column
sqlite> create table ABC(A TEXT, B VARCHAR);
sqlite> pragma table_info(ABC);
cid name type notnull dflt_value pk
---------- ---------- ---------- ---------- ---------- ----------
0 A TEXT 0 0
1 B VARCHAR 0 0

Answer 2

Execute the query:
PRAGMA table_info( your_table_name );
Documentation

Answer 3

PRAGMA table_info( your_table_name ); doesn't work in HTML5 SQLite.
Here is a small HTML5 SQLite JavaScript Snippet which gets the column names from your_table_name even if its empty. Hope its helpful.
tx.executeSql('SELECT name, sql FROM sqlite_master WHERE type="table" AND name = "your_table_name";', [], function (tx, results) {
var columnParts = results.rows.item(0).sql.replace(/^[^\(]+\(([^\)]+)\)/g, '$1').split(',');
var columnNames = [];
for(i in columnParts) {
if(typeof columnParts[i] === 'string')
columnNames.push(columnParts[i].split(" ")[0]);
}
console.log(columnNames);
///// Your code which uses the columnNames;
});

Answer 4

Execute this query
select * from (select "") left join my_table_to_test b on -1 = b.rowid;
You can try it at online sqlite engine

Answer 5

The PRAGMA statement suggested by #pragmanatu works fine through any programmatic interface, too. Alternatively, the sql column of sqlite_master has the SQL statement CREATE TABLE &c &c that describes the table (but, you'd have to parse that, so I think PRAGMA table_info is more... pragmatic;-).

Answer 6

If you are suing SQLite 3.8.3 or later (supports the WITH clause), this recursive query should work for basic tables. On CTAS, YMMV.
WITH
Recordify(tbl_name, Ordinal, Clause, Sql)
AS
(
SELECT
tbl_name,
0,
'',
Sql
FROM
(
SELECT
tbl_name,
substr
(
Sql,
instr(Sql, '(') + 1,
length(Sql) - instr(Sql, '(') - 1
) || ',' Sql
FROM
sqlite_master
WHERE
type = 'table'
)
UNION ALL
SELECT
tbl_name,
Ordinal + 1,
trim(substr(Sql, 1, instr(Sql, ',') - 1)),
substr(Sql, instr(Sql, ',') + 1)
FROM
Recordify
WHERE
Sql > ''
AND lower(trim(Sql)) NOT LIKE 'check%'
AND lower(trim(Sql)) NOT LIKE 'unique%'
AND lower(trim(Sql)) NOT LIKE 'primary%'
AND lower(trim(Sql)) NOT LIKE 'foreign%'
AND lower(trim(Sql)) NOT LIKE 'constraint%'
),
-- Added to make querying a subset easier.
Listing(tbl_name, Ordinal, Name, Constraints)
AS
(
SELECT
tbl_name,
Ordinal,
substr(Clause, 1, instr(Clause, ' ') - 1),
trim(substr(Clause, instr(Clause, ' ') + 1))
FROM
Recordify
WHERE
Ordinal > 0
)
SELECT
tbl_name,
Ordinal,
Name,
Constraints
FROM
Listing
ORDER BY
tbl_name,
lower(Name);