Use IN in Java

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Use IN
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How can you encourage early breakout Avoid the use of syntax like = (SELECT
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<query>) Use = ANY (SELECT <query>) expressions or the commoner shorthand IN (SELECT <query>) instead, because = (SELECT <query>) has to chug through the whole inner loop before it's certain that there is only one match In other words, don't use forms like Query #1 Replace them with forms like Query #2 or Query #3: Query #1 /* avoid this */ SELECT column1 FROM Table1 WHERE column1 = (SELECT column1 FROM Table2) Query #2 /* use this */ SELECT column1 FROM Table1 WHERE column1 = ANY (SELECT column1 FROM Table2) Query #3 /* or use this */ SELECT column1 FROM Table1 WHERE column1 IN (SELECT column1 FROM Table2) Join: SELECT MIN(Table1column1) FROM Table1, Table2 WHERE Table1column1 = Table2column1 Subquery: SELECT MIN(Table1column1) FROM Table1 WHERE Table1column1 IN (SELECT Table2column1 FROM Table2) GAIN: 7/7
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Portability MySQL doesn't support subqueries The gain on all tests shown in this chapter is for only seven DBMSs
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The result of the test was that the subquery was faster with every DBMS tested Here are some other arguments in the join-versus-subquery debate:
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Pro join arguments
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People who prefer joins over subqueries pose the following arguments:
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If the outer query's WHERE clause contains multiple ANDed/ORed conditions, the optimizer can arrange them better if everything is on one level that is, if you flatten to a join Optimizers tend to optimize each query separately Some DBMSs (eg, Oracle) can parallelize joins better than they can parallelize subqueries When you do a join, data from both tables can be in the final select list This isn't true for subqueries Simply because people think joins are better, they use them more, and therefore DBMS vendors work harder to make joins better It's a self-fulfilling prophecy!
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Pro subquery arguments
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On the other side, people who prefer subqueries over joins argue these points:
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Comparison rules might be more relaxed in subqueries For example, SMALLINTs can be looked up in INTEGER subqueries without major penalties Subqueries can contain GROUP BY, HAVING, and set functions It's more cumbersome to join when such syntax exists, especially if the syntax is hidden within a view definition Subqueries can appear in UPDATE statements Most DBMSs can't update a join Subqueries that use a comparison operator without ANY, and subqueries that use NOT EXISTS within NOT EXISTS (the "relational divide" problem), are not expressible using joins with classic syntax In mature DBMSs, vendors utilize subquery optimizations similar to those that are used to optimize joins For example, IBM was once unable to take advantage of indexes when doing in-to-out, but now IBM does use indexes in such situations
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Flattening
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To flatten a query means to make everything one level Based on our observations so far, there are clearly times when you should flatten a subquery In this section, we'll discuss how the DBMS can flatten automatically, but first let's establish how you can do it yourself To flatten IN Here's an example of a subquery using IN, and the flattened analogous query:
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Subquery with IN: SELECT * FROM Table1 WHERE Table1column1 IN (SELECT Table2column1 FROM Table2 WHERE Table2column1 = 5) Flattened: SELECT Table1* FROM Table1, Table2 WHERE Table1column1 = Table2column1 AND Table2column1 = 5
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This simple flattening seems like it should be equivalent to the IN query, but it has a flaw that can be seen if Table1 has these three rows: {1, 5, 5} and Table2 has these three rows: {2, 5, 5} Notice that the subquery-based SELECT would correctly return two rows: {5, 5}, but the flattened
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SELECT would return four rows: {5, 5, 5, 5} Notice further that if the flattened SELECT is "corrected" by using SELECT DISTINCT instead of just SELECT, the join would return only one row: {5} In other words, flattening can cause too many duplicate rows and if you try to get rid of the duplicates, you can end up with too few duplicate rows Sometimes you'll see this flaw if the DBMS flattens automatically (By the way, DISTINCT can be expensive so don't use it if Table2column1 is uniquely indexed) To flatten NOT IN Here's an example of a subquery using NOT IN, and the flattened equivalent query:
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Subquery with NOT IN: SELECT Table1column1 FROM Table1 WHERE Table1column1 NOT IN (SELECT Table2column1 FROM Table2) Flattened equivalent: SELECT Table1column1 FROM Table1 LEFT JOIN Table2 ON Table1column1 = Table2column1 WHERE Table2column1 IS NULL
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This type of flattening is rare, and has two flaws: it assumes that ON happens before WHERE as in the SQL Standard, and it assumes that Table2column1 has no NULLs (the WHERE Table2column1 IS NULL clause is meant to find dummy rows produced by the outer join) When is flattening automatic The rules vary slightly between DBMSs and often seem to reflect some foible in the DBMS rather than some strategy Amalgamating the rules followed by the various DBMSs, we can say that a flattening is almost certain to happen to subqueries in this scenario:
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Flattening is legal that is, the subquery isn't in an UPDATE statement, contains no implied groupings, and so on The outer query does not have any of {<>, NOT EXISTS, AND, OR, CASE} directly before the subquery The inner query does not have a join or another inner query There is not a one-to-many relation (The DBMS might determine that by finding a UNIQUE index on the subquery column)
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If you think the DBMS is likely to flatten incorrectly, you can discourage it by changing the conditions If you think the DBMS is likely to flatten correctly, then you face the question Why code with a subquery in the first place If it's going to be a join anyway, it would be clearer to code it as a join In other words, if flattening is automatic, that's a reason to flatten manually! We think that's taking things a bit too far Better to let the DBMS decide, and worry yourself about the portability Keep in mind that the original subquery code is there because somebody thought it was better
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