How do I do a SQL JOIN on 3 tables? - mysql

Question

I am using MySQL and I am trying to get data out of three different tables but am not sure of the syntax that I should use.
I have a table called full_daily_data_1 with the following fields
Symbol
Trade_Date
Day_Open
Day_High
I also have a table called custom_indices_xref with the following fields:
symbol
Custom_Index_Name
And a table called daily_index_weightings with the following fields:
Custom_Index_Name
Symbol
Trade_date
combo_weighting
Right now, I have this as a select statement to get the data I need out of two tables:
SELECT
Symbol,
Trade_Date,
Day_Open,
Day_High
FROM
full_daily_data_1
WHERE
trade_date >= '2012/01/01' AND
trade_date <= '2012/01/31' AND
symbol in (SELECT symbol from custom_indices_xref WHERE Custom_Index_Name = 'Agricultural-Chemical-and-Fertilizer-Stocks');
But what I want is the following data for each symbol for each date in the date range
symbol
Day_Open
Day_High,
Custom_Index_Name
Symbol
Trade_date
combo_weighting
So basically, needing to add combo_weighting from the daily_index_weightings table for the selected days and symbols. What should my SQL statement look like to accomplish this?
I tried this, but I get a SQL syntax error, so not sure what I'm doing wrong:
SELECT
full_daily_data_1.Symbol,
full_daily_data_1.Trade_Date,
full_daily_data_1.Day_Open,
full_daily_data_1.Day_High,
full_daily_data_1.Day_Low,
daily_index_weightings.combo_weighting
FROM
full_daily_data_1
WHERE
trade_date >= '2012/01/01' AND
trade_date <= '2012/01/31' AND
Symbol in (SELECT symbol from custom_indices_xref WHERE Custom_Index_Name = 'Agricultural-Chemical-and-Fertilizer-Stocks')
JOIN
daily_index_weightings ON
daily_index_weightings.symbol = full_daily_data_1.Symbol AND
daily_index_weightings.Trade_Date = full_daily_data_1.Trade_Date ;

Answer 1

SELECT fdd.Symbol, fdd.Trade_Date, fdd.Day_Open, fdd.Day_High, fdd.Day_Low, diw.combo_weighting
FROM full_daily_data_1 fdd
INNER JOIN custom_indicies_xref cix ON fdd.symbol=cix.symbol
INNER JOIN daily_index_weighings diw ON fdd.symbol = diw.symbol
WHERE
trade_date >= '2012/01/01' AND
trade_date <= '2012/01/31' AND
cix.Custom_Index_Name = 'Agricultural-Chemical-and-Fertilizer-Stocks'
The INNER JOINS could be changed to LEFT JOIN if required.
Alternatively, since your second and third tables also match the Custom_Index_Name fields (which is bad design, by the way):
SELECT fdd.Symbol, fdd.Trade_Date, fdd.Day_Open, fdd.Day_High, fdd.Day_Low, diw.combo_weighting
FROM full_daily_data_1 fdd
INNER JOIN (custom_indicies_xref cix ON fdd.symbol=cix.symbol
INNER JOIN daily_index_weighings diw ON cix.Custom_Index_Name = diw.Custom_Index_Name)
ON fdd.symbol = cix.symbol
WHERE
trade_date >= '2012/01/01' AND
trade_date <= '2012/01/31' AND
cix.Custom_Index_Name = 'Agricultural-Chemical-and-Fertilizer-Stocks'

Answer 2

Your sql clauses are out of order and you should use alias for your table names, just makes it easier to read. Another thing I noticed is your fields in your WHERE clause are ambiguous.
SELECT
FDD.Symbol,
FDD.Trade_Date,
FDD.Day_Open,
FDD.Day_High,
FDD.Day_Low,
DIW.combo_weighting
FROM
full_daily_data_1 FDD
JOIN
daily_index_weightings DIW ON
DIW.symbol = FDD.Symbol AND
DIW.Trade_Date = FDD.Trade_Date ;
WHERE
FDD.trade_date >= '2012/01/01' AND
FDD.trade_date <= '2012/01/31' AND
FDD.Symbol in
(SELECT symbol from custom_indices_xref WHERE Custom_Index_Name = 'Agricultural-Chemical-and-Fertilizer-Stocks')