Postgresql if then else8/17/2023 Now, if we could just get a few horrifying error messages from SQL Server to do the same. It all seems kind of annoying until it is 3 in the morning and you get an error message that you don’t have a value to start looking for in your source query. So using a common variable that you trust to be set by a literal just feels safer, and adding on error message stuff is less dangerous in the long run. A customer named ‘’, possible if you do not have constraints to prevent empty data. CustomerName is tricky too, because you need to make sure that you pick a value that CustomerName can never be. Rowcount is not a bad choice, but it is easy for some other coder to inadvertently mess up fetching the rowcount, because you have to get in the very next statement or it can be cleared. Note that I still used the variable, rather than using rowcount, or the name of the customer. If you have used a CTE in your query, this can sometimes be tricky as they cannot be used in a conditional like IF EXISTS (queryWithCTE).įor example, say your query was the following (using WideWorldImporters), where you want to make sure that a customer only ordered one size of product (this predicate is clearly senseless (and no data meets this requirement), but uses it WWI data which is simple and something that everyone can easily attain and fiddle with.) So, you might write the following query: The problem is, often you want to write a query to look for bad data, fix the bad data in the table, then use the same base query in a procedure/trigger or testing/validation code. Code that often needed a tangle of temp tables could be now be done in a single query (Derived tables can be used too, but I can’t remember when derived tables started in SQL Server, but it may have been 2005, or perhaps 2000). One of the most exciting features of SQL Server 2005 was the inclusion of Common Table Expressions (CTE). But I had this come up in some work I was doing and wanted to put it down in a blog. WHERE CASE WHEN x 0 THEN y/x > 1.I am in the middle of working on my DB Design conference that occurs in a bit over a week from now. For example, this is a possible way of avoiding a division-by-zero failure: SELECT. The example above can be written using the simple CASE syntax: SELECT a,Ī CASE expression does not evaluate any subexpressions that are not needed to determine the result. This is similar to the switch statement in C. If no match is found, the result of the ELSE clause (or a null value) is returned. The first expression is computed, then compared to each of the value expressions in the WHEN clauses until one is found that is equal to it. There is a "simple" form of CASE expression that is a variant of the general form above: CASE expression The data types of all the result expressions must be convertible to a single output type. If the ELSE clause is omitted and no condition is true, the result is null. If no WHEN condition yields true, the value of the CASE expression is the result of the ELSE clause. NULLIF in postgreSQL is an in-built conditional function that takes two arguments and returns null if two arguments are equal. If the condition's result is not true, any subsequent WHEN clauses are examined in the same manner. If the condition's result is true, the value of the CASE expression is the result that follows the condition, and the remainder of the CASE expression is not processed. Each condition is an expression that returns a boolean result. The SQL CASE expression is a generic conditional expression, similar to if/else statements in other programming languages: CASE WHEN condition THEN resultĬASE clauses can be used wherever an expression is valid.
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