Forgotten rows
In this example you are shown how to include the rows your JOIN function automatically leaves out as it does not perform the join when one of the values is 0.
This is done by adding a either a LEFT JOIN or a UNION making the join also to reveal the rows with a count of 0.
Table 1 shows the results without a LEFT JOIN and table 2 shows the results we obtain with the LEFT JOIN
| name | COUNT(custid) |
|---|---|
| Betty | 2 |
| Janette | 1 |
| name | COUNT(custid) |
|---|---|
| Betty | 2 |
| Janette | 1 |
| Robert | 0 |
DROP TABLE customer;
DROP TABLE invoice;
CREATE TABLE customer(
id INTEGER,
name VARCHAR(20));
INSERT INTO customer VALUES (1,'Betty');
INSERT INTO customer VALUES (2,'Robert');
INSERT INTO customer VALUES (3,'Janette');
CREATE TABLE invoice(
invoiceno INTEGER,
whn DATE,
custid INTEGER,
cost INTEGER );
INSERT INTO invoice VALUES (1,'2006-11-01',1,100);
INSERT INTO invoice VALUES (2,'2006-11-05',1,500);
INSERT INTO invoice VALUES (3,'2006-11-11',3,200);
The following query will only give two rows as the JOIN function automatically does not include rows with a count of 0.
SELECT name, COUNT(custid) FROM customer JOIN invoice ON (id=custid) GROUP BY name
In order to obtain the rows where the count from the query is 0 a
LEFT JOIN or a UNION can be used.
SELECT name, COUNT(custid)
FROM customer LEFT JOIN invoice ON (id=custid)
GROUP BY name
Hack 10 Converting subqueries into joins
Hack 11 Converting aggregate subqueries into joins
Hack 16 Search for a String across columns
Hack 24 Multiply Across a Result Set
Hack 25.5 Splitting and combining columns
Hack 26 Include the rows your JOIN forgot
Hack 30 Calculate the maximum/minimum of two fields
Hack 33 Get values and subtotals in one shot
Hack 50 Combine tables containing different data
Hack 51/52 Display rows as columns
Hack 55 Import Someone Else's Data
Hack 62 Issue Queries Without Using a Table
Hack 63 Generate rows without tables
Hack 72 Extract a subset of the results
Hack 78 Break it down by Range
Hack 88 Test two values from a subquery
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