![]() If you prefix the query from above with EXPLAIN QUERY PLAN you can verify that SQLite will need to perform a full scan of the table just to find the “Kotlin” tag: EXPLAIN QUERY PLAN SELECT * FROM tags WHERE title = "Kotlin" What if we don’t necessarily know the rowid for the tag we need, but instead wanted to search for the “Kotlin” tag: SELECT * FROM tags WHERE title = "Kotlin" Īs of the current state of our schema, SQLite will need to iterate one-by-one through the whole table until it finds the record where title = "Kotlin" holds true: an O(n) operation. So far, we’ve only been talking about primary keys. The algorithms SQLite uses to determine how to execute a query are grouped up into a module called the “Query Planner”, and performing that determination is called “query planning”. ![]() When we run SELECT title FROM tags WHERE rowid = 6 SQLite does a binary search, first looking at the middle record with rowid = 9/2 = 4, realizes that’s too low, and tries rowid = (9-4)/2 + 4 = 2 + 4 =6 and bingo! That took 2 steps, and is a lot faster than iterating through each record until it finds the 6th record (which clearly would’ve taken 6 steps).īut wait, why couldn’t SQLite just immediately jump to the 6th record? Well, even though the rowid is auto-incrementing and unique, we could’ve deleted rows in the table and thereby created gaps where the 6th record might not be the record with rowid = 6. The NULLS FIRST option specifies that the NULLs will appear at the beginning of the result set while the NULLS LAST option place NULLs at the end of the result set.This table doesn’t explicitly specify a PRIMARY KEY column, so rowid is used instead. SQLite 3.30.0 added the NULLS FIRST and NULLS LAST options to the ORDER BY clause. It means that NULLs will appear at the beginning of the result set if you use ASC or at the end of the result set when you use DESC. ![]() ![]() When it comes to sorting, SQLite considers NULL to be smaller than any other value. NULL is even cannot be compared with itself NULL is not equal to itself so NULL = NULL always results in false. Simply put, if the two pieces of information are unknown, you cannot compare them. NULL is special because you cannot compare it with another value. Instead of using a special value to indicate that the information is missing, NULL is used. However, both of these values do not clearly show that the birthday is unknown. To represent the unknown birthday information in the database, you may use a special date like or an '' empty string. At the time of saving the artist’s record, you don’t have the birthday information. Suppose you want to store the birthday of an artist in a table. It denotes that the information missing or the data is not applicable. The number 3 and 2 refers to the AlbumId and Milliseconds in the column list that appears in the SELECT clause. ORDER BY 3, 2 Code language: SQL (Structured Query Language) ( sql ) Suppose, you want to get data from name, milliseconds, and album id columns, you use the following statement: Let’s take the tracks table in the sample database for the demonstration. You can sort the result set using a column that does not appear in the select list of the SELECT clause. Then, it sorts the sorted rows using the second column, and so on. In other words, the ORDER BY clause sorts the rows using the first column in the list. The ORDER BY clause sorts rows using columns or expressions from left to right. In case you want to sort the result set by multiple columns, you use a comma (,) to separate two columns. In other words, it sorts the result set in the ascending order by default. If you don’t specify the ASC or DESC keyword, SQLite sorts the result set using the ASC option. In this syntax, you place the column name by which you want to sort after the ORDER BY clause followed by the ASC or DESC keyword. It allows you to sort the result set based on one or more columns in ascending or descending order. The ORDER BY clause comes after the FROM clause. Column_2 DESC Code language: SQL (Structured Query Language) ( sql )
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