mysql LEFT JOIN与其他表的条件

Question

I have two mysql tables:

table-1: table_item

╔════╦═══════╦════════╦══════════╗
║ id ║ catid ║ itemid ║ itemname ║
╠════╬═══════╬════════╬══════════╣
║  1 ║   1   ║   1    ║ Pen      ║
║  2 ║   1   ║   2    ║ Pencil   ║
║  3 ║   1   ║   3    ║ Sharpner ║
║  4 ║   2   ║   4    ║ Book     ║
║  5 ║   2   ║   5    ║ Notebook ║
║  6 ║   3   ║   6    ║ Pant     ║
║  7 ║   4   ║   7    ║ Shirt    ║
╚════╩═══════╩════════╩══════════╝

table-2: bid-2015

╔════╦════════╦══════╦══════╗
║ id ║ itemid ║ year ║ rate ║
╠════╬════════╬══════╬══════╣
║  1 ║   1    ║ 2015 ║ 3.0  ║
║  2 ║   2    ║ 2015 ║ 5.0  ║
║  3 ║   5    ║ 2015 ║ 7.0  ║
║  4 ║   1    ║ 2016 ║ 3.5  ║
║  5 ║   5    ║ 2016 ║ 7.8  ║
║  6 ║   7    ║ 2016 ║ 20.0 ║
╚════╩════════╩══════╩══════╝

I want to result like below:

result table: for the year 2015

╔══════════╦══════╦══════╗
║ itemname ║ rate ║ year ║
╠══════════╬══════╬══════╣
║ Pen      ║ 3.0  ║ 2015 ║
║ Pencil   ║ 5.0  ║ 2015 ║
║ Sharpner ║ null ║ null ║
╚══════════╩══════╩══════╝

to get the above result table if I run the below query:

SELECT i.itemname, b.rate, b.year
FROM table_item i LEFT JOIN bid-2015 b ON i.itemid=b.itemid
WHERE i.catid=1 AND b.year=2015

then it gives me following output:

╔══════════╦══════╦══════╗
║ itemname ║ rate ║ year ║
╠══════════╬══════╬══════╣
║ Pen      ║ 3.0  ║ 2015 ║
║ Pencil   ║ 5.0  ║ 2015 ║
║ Notebook ║ 7.0  ║ 2015 ║
╚══════════╩══════╩══════╝

How can I LEFT JOIN those tables with conditions so that it can output first table?

Answer 1

A LEFT JOIN is like INNER JOIN except that it will return each record from a at least once, substituting missing fields from b with NULL values, if there are no actual matching records.

The WHERE condition, however, is evaluated after the LEFT JOIN so the query above checks column after it had been joined. No NULL value can satisfy an equality condition, so the records from a without corresponding record from b will unavoidably be filtered out.

Essentially, this query is an INNER JOIN, only less efficient.

To match only the records with b.column = 'something' (while still returning all records from a), this condition should be moved into ON clause:

SELECT i.itemname,b.rate,b.year 
from table_item i 
LEFT JOIN bid-2015 b 
ON i.itemid=b.itemid
AND i.catid=1 AND b.year=2015