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ow that you know how to build and work with SQL Server objects, and insert, update, and delete data as well as retrieve it, we can move on to more of the T-SQL essentials required to complete your programming knowledge. Potentially the most important area covered by this chapter is error handling. After all, no matter how good your code is, if it cannot cope when an error occurs, then it will be hard to keep the code stable and reliable. There will always be times that the unexpected happens, either from strange input data to something happening in the server. However, this is not the only area of interest. You will be looking at joining tables together, performing aggregations of data, and grouping data together. Finally, there will be times that you wish to hold data either in a variable or within a table that you only want to exist for a short period. Quite a great deal to cover, but this chapter and the next will be the stepping stones that move you from a novice to a professional developer. This chapter will therefore look at the following: Joining two or more tables to see more informational results Having a method of storing information on a temporary basis via variables How to hold rows of information in a nonpermanent table How to aggregate values Organizing output data into groups of relevant information Returning unique and distinct values Looking at and using system functions Error handling: how to create your own errors, trap errors, and make code secure
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Throughout this book, the SELECT and UPDATE statements have only dealt with and covered the use of one table. However, it is possible to have more than one table within our SELECT or UPDATE statement, but we must keep in mind that the more tables included in the query, the more detrimental the effect on the query s performance. When we include subsequent tables, there must be a link of some sort between the two tables, known as a join. A join will take place between at least one column in one table and a column from the joining table. The columns involved in the join do not have to be in any key within the tables involved in the join. However, this is quite uncommon, and if you do find you are joining tables, then there is a high chance that a relationship exists between them, which would mean you do require a primary key and a foreign key. This was covered in 3. It is possible that one of the columns on one side of the join is actually a concatenation of two or more columns. As long as the end result is one column, this is acceptable. Also, the two columns
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CHAPTER 11 T-SQL ESSENTIALS
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that are being joined do not have to have the same name, as long as they both have similar data types. For example, you can join a char with a varchar. What is not acceptable is that one side of the JOIN names a column and on the other side is a variable or literal that is really a filter that would be found in a WHERE statement. Joining two tables together can become quite complicated. The most basic join condition is a straight join between two tables, which is called an INNER JOIN. An INNER JOIN joins the two tables, and where there is a join of data using the columns from each of the two tables, then the data is returned. For example, if there is a share in the shares table that has no price and you are joining the two tables on the share ID, then you would only see output where there is a share with a share price. You will see this in action in this chapter. It is possible to return all the rows from one table where there is no join. This is known as an OUTER JOIN. Depending on which table you want the rows always to be returned from, this will either be a LEFT OUTER JOIN or a RIGHT OUTER JOIN. Taking our shares example, we could use an OUTER JOIN so that even when there is no share price, we can still list the share. This example will also be demonstrated later in this chapter. The final type of join is the scariest and most dangerous join. If you wish for every row in one table to be joined with every row in the joining table, then you would use a CROSS JOIN. So if you had 10 rows in one table and 12 rows in the other table, you would see returned 120 rows of data (10 12). As you can imagine, this type of join just needs two small tables to produce even a large amount of output. Although not the most helpful of syntax demonstrated within the book, the syntax for joining two tables is as follows: FROM tablea [FULL[INNER|OUTER|CROSS]] JOIN tableb {ON tableb.column1 = tablea.column2 {AND|OR tableb.column...}} The best way to look at the syntax is within a described example. We will use two tables to demonstrate the inner join in this example: ShareDetails.Shares and ShareDetails.SharePrices. Joining two tables could not be simpler. All the columns in both tables are available to be returned through the query, so we can list the columns desired as normal. However, if there are two columns of the same name, they must be prefixed with the name, or the alias name, of the table from which the information is derived.
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