SQL Server performs sort, intersect, union, and difference operations using in-memory sorting and hash join technology. Using this type of query plan, SQL Server supports vertical table partitioning. SQL Server employs four types of physical join operations to carry out the logical join operations:.
By using joins, you can retrieve data from two or more tables based on logical relationships between the tables. Joins indicate how SQL Server should use data from one table to select the rows in another table.
Outer joins and cross joins can be specified in the FROM clause only. Specifying the join conditions in the FROM clause helps separate them from any other search conditions that may be specified in a WHERE clause, and is the recommended method for specifying joins.
The following is an example of a FROM clause join specification:. When multiple tables are referenced in a single query, all column references must be unambiguous. Any column name that is duplicated between two or more tables referenced in the query must be qualified with the table name. All references to the Vendor columns in the example are qualified. When a column name is not duplicated in two or more tables used in the query, references to it do not have to be qualified with the table name.
This is shown in the previous example. Such a SELECT clause is sometimes difficult to understand because there is nothing to indicate the table that provided each column.
The readability of the query is improved if all columns are qualified with their table names. The readability is further improved if table aliases are used, especially when the table names themselves must be qualified with the database and owner names. The following is the same example, except that table aliases have been assigned and the columns qualified with table aliases to improve readability:.
The previous examples specified the join conditions in the FROM clause, which is the preferred method. For example, in a three-table join, only one table can be used to bridge from one of the other tables to the third table, and none of the columns from the middle table have to be referenced in the select list.
This is also called an anti semi join. When SQL Server processes joins, the Query Optimizer chooses the most efficient method out of several possibilities of processing the join. This includes choosing the most efficient type of physical join, the order in which the tables will be joined, and even using types of logical join operations that cannot be directly expressed with Transact-SQL syntax, such as semi joins and anti semi joins.
The physical execution of various joins can use many different optimizations and therefore cannot be reliably predicted. For more information on semi joins and anti semi joins, see Showplan Logical and Physical Operators Reference. Columns used in a join condition are not required to have the same name or be the same data type. However, if the data types are not identical, they must be compatible, or be types that SQL Server can implicitly convert.
If the data types cannot be implicitly converted, the join condition must explicitly convert the data type using the CAST function. For more information about implicit and explicit conversions, see Data Type Conversion Database Engine. Most queries using a join can be rewritten using a subquery a query nested within another query , and most subqueries can be rewritten as joins. For more information about subqueries, see Subqueries.
Tables cannot be joined directly on ntext, text, or image columns. The nested loops join, also called nested iteration , uses one join input as the outer input table shown as the top input in the graphical execution plan and one as the inner bottom input table. The outer loop consumes the outer input table row by row.
The inner loop, executed for each outer row, searches for matching rows in the inner input table. In the simplest case, the search scans an entire table or index; this is called a naive nested loops join. If the search exploits an index, it is called an index nested loops join. If the index is built as part of the query plan and destroyed upon completion of the query , it is called a temporary index nested loops join.
All these variants are considered by the Query Optimizer. A nested loops join is particularly effective if the outer input is small and the inner input is preindexed and large. In many small transactions, such as those affecting only a small set of rows, index nested loops joins are superior to both merge joins and hash joins. In large queries, however, nested loops joins are often not the optimal choice.
The presence of this optimization in a given plan may not be very obvious when analyzing an execution plan, given the sort itself is a hidden operation. If the two join inputs are not small but are sorted on their join column for example, if they were obtained by scanning sorted indexes , a merge join is the fastest join operation.
If both join inputs are large and the two inputs are of similar sizes, a merge join with prior sorting and a hash join offer similar performance. However, hash join operations are often much faster if the two input sizes differ significantly from each other.
The merge join requires both inputs to be sorted on the merge columns, which are defined by the equality ON clauses of the join predicate. Table 1. HASH join. Understanding Three-table Joins in Oracle. Comments is there a mistake in the diagramm — the hash table is formed from small table which is dept and join key is use to probe the larger table employee.
Hi, One quick question. Thanks, Sanu. Good one.. Because SQL Server Optimizer performs this dynamic operation during execution time and not during compilation time, sometimes the values you see in the execution plan are incorrect. In some cases, you could compare a hash join and a nested loops join, see that according to the execution plans the nested loop is more expensive in terms of logical reads etc. Microsoft has an interesting page in Books Online that describes further Hash Join sub-types and additional aspects about them.
Log in or register to rate. I've grown up reading Tom Clancy and probably most of you have at least seen Red October, so this book caught my eye when browsing used books for a recent trip.
It's a fairly human look at what's involved in sailing on a Trident missile submarine Question: Can a SQL instance be used as the witness for a database mirroring setup? This question was sent to me via email. My reply follows.
Can a SQL instance be used as the witness for a database mirroring setup? Databases to be mirrored are currently running on SQL instances but will be upgraded to SQL in the near future.
In which Phil illustrates an old trick using STUFF to intert a number of substrings from a table into a string, and explains why the technique might speed up your code You may want to read Part 1 , Part 2 , and Part 3 before continuing. This time around I'd like to talk about social networking.
We'll start with social networking. Facebook, MySpace, and Twitter are all good examples of using technology to let I've got a few more thoughts on the topic this week, and I look forward to your comments.
0コメント