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CHAPTER 11 T-SQL ESSENTIALS
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Try It Out: Joining Two Tables
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1. The first join we will look at is the INNER JOIN. This is where we have two tables and we want to list all the values where there is a join. In this case, we want to list all the shares where there is a share price, and we want to see every share price for that share. Notice that we don t need to define the word INNER. This is presumed if nothing else is specified. Also take note that, like columns, we have defined aliases for the table names. This makes prefixing columns easier. We are joining the two tables on ShareId, as this is the linking column between the two tables. Enter the following code: SELECT s.ShareDesc,sp.Price,sp.PriceDate FROM ShareDetails.Shares s JOIN ShareDetails.SharePrices sp ON sp.ShareId = s.ShareId 2. Once you have executed the code, you should see the output that appears in Figure 11-1. There is no output for ShareIds 3, 4, and 5, as they have no share price.
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Figure 11-1. First inner join 3. We can take this a stage further and filter the rows to only list the share price row that matches the CurrentPrice in the ShareDetails.Shares table. This could be done by filtering the data on a WHERE statement, and from a performance perspective it would be better, as neither of these columns are within an index and there could be a large number of rows for ShareDetails.SharePrices for each share as time goes on; but for this example, it demonstrates how to add a second column for the join. SELECT FROM JOIN AND s.ShareDesc,sp.Price,sp.PriceDate ShareDetails.Shares s ShareDetails.SharePrices sp ON sp.ShareId = s.ShareId sp.Price = s.CurrentPrice
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4. Execute the preceding code, which will return two rows as shown in Figure 11-2. As you can see, an INNER JOIN is very straightforward.
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Figure 11-2. Inner join with multiple join columns
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CHAPTER 11 T-SQL ESSENTIALS
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5. The next join we look at is an OUTER JOIN, more specifically a LEFT OUTER JOIN. In this instance, we want to return all the rows in the left table, whether there is any data in the right table or not. The left table in this case is the ShareDetails.Shares, table as it is the left named table of the two we are concerned with. Enter the following code: SELECT s.ShareDesc,sp.Price,sp.PriceDate FROM ShareDetails.Shares s LEFT OUTER JOIN ShareDetails.SharePrices sp ON sp.ShareId = s.ShareId 6. Once you execute this code, you should see the missing shares from the previous example listed, as you see in Figure 11-3. Notice that where no data exists in the ShareDetails.SharePrices table, the values are displayed as NULL. OUTER JOINS are a good tool when checking other queries. For example, the results in Figure 11-3 demonstrate that quite rightly, the bottom three shares should have been missing in the first example, as they did not meet our criteria. This may not be so obvious when there are large volumes of data though.
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Figure 11-3. Left outer join 7. To get around this problem, we can add a WHERE statement that will list those shares that do not have an item in ShareDetails.SharePrices. This is one method of achieving our goal. We will look at the other later in the chapter when we examine EXISTS. We know that when there is a missing share price, Price and PriceDate will be NULL. It is also necessary to know that Price cannot have a NULL value inserted in any rows of data. If it could, then we would need to use another method, such as EXISTS. SELECT s.ShareDesc,sp.Price,sp.PriceDate FROM ShareDetails.Shares s LEFT OUTER JOIN ShareDetails.SharePrices sp ON sp.ShareId = s.ShareId WHERE sp.Price IS NULL 8. This time we will only have three rows returned, as you see in Figure 11-4.
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