Syntax is the set of rules by which the elements of a language are correctly combined. Sql syntax is based on English syntax, and uses many of the same elements as Visual Basic for Applications (VBA) syntax. For example, a simple sql statement that retrieves a list of last names for contacts whose first name is Mary might resemble this: select last_Name, from contacts, where first_Name 'mary note: sql is not only used for manipulating data, but also for creating and altering. The part of sql that is used for creating and altering database objects is called data-definition language (DDL). This topic does not cover ddl. For more information, see the article.
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In this article, what is sql? Basic york sql clauses: select, from, and where. Sorting the results: order by, working with summarized data: group by and having. Combining query results: union, what is sql? Sql is a computer language for working with sets of facts and the relationships between them. Relational database programs, such as Microsoft Office Access, use sql to work with data. Unlike many computer languages, sql is not difficult to read and understand, even for a novice. Like many computer languages, sql is an international standard that is recognized by standards bodies such. You use sql to describe sets of data that can help you answer questions. When you use sql, you must use the correct syntax.
Try it out see the answer In the previous examples, data was displayed vertically, but in some instances, you might want to show data horizontally. This is known as pivoting (like a pivot table in Excel). Lets take the following query: select case when year 'fr' then 'fr' when year 'so' then 'so' when year 'jr' then 'jr' when year 'sr' then 'sr' else 'no year Data' end as year_group, count(1) as count from llege_football_players margaret group by 1 And re-orient. Sharpen your sql skills Write a query that displays the number of players in each state, with fr, so, jr, and sr players in separate columns and another column for the total number of players. Order results such that states with the most players come first. Try it out see the answer Write a query that shows the number of players at schools with names that start with A through m, and the number at schools with names starting with n -. Try it out see the answer.
Heres an example of counting multiple conditions in one query: select case when year 'fr' then 'fr' when year 'so' then 'so' when year 'jr' then 'jr' when year 'sr' then 'sr' else 'no year Data' end as year_group, count(1) as count from llege_football_players group. Alternatively, you can use the columns alias in the group by clause like this: select case when year 'fr' then 'fr' when year 'so' then 'so' when year 'jr' then 'jr' when year 'sr' then 'sr' else 'no year Data' end as year_group, count(1). Its often helpful to write a query containing the case statement first and run it on its own. Using the previous example, you might first write: select case when year 'fr' then 'fr' when year 'so' then 'so' when year 'jr' then 'jr' when year 'sr' then 'sr' else 'no year Data' end as year_group, * from llege_football_players The above query will show. From there, you can replace the * with an aggregation and add a group by clause. Try this process if you struggle with either of the following practice problems. Write a query that counts the number of 300lb players for each of the following regions: West coast (ca, or, wa texas, and Other (Everywhere else). Try it out see the answer Write a query that calculates the combined weight of all underclass players (FR/SO) in California as well as the combined weight of all upperclass players (JR/SR) in California.
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Keep in mind that the answer we provide is only one of many possible answers, since you could divide players' heights in many ways. Try it out, see the answer, you can also string together multiple conditional statements with and and or the same way you might in a help where clause: select player_name, case when year 'fr' and position 'wr' then 'frosh_wr' else null end as sample_case_statement from llege_football_players. Else is an optional component. You can make any conditional statement using any conditional operator (like where ) between when and then. This includes stringing together multiple conditional statements using and and. You can include multiple when statements, as well as an else statement to deal with any unaddressed conditions. Write a jealousy query that selects all columns from llege_football_players and adds an additional column that displays the player's name if that player is a junior or senior.
Try it out see the answer cases slightly more complicated and substantially more useful functionality comes from pairing it with aggregate functions. For example, lets say you want to only count rows that fulfill a certain condition. Since count ignores nulls, you could use a case statement to evaluate the condition and produce null or non-null values depending on the outcome: select case when year 'fr' then 'fr' else 'not fr' end as year_group, count(1) as count from llege_football_players group by case. You could do that—it would look like this: select count(1) as fr_count from llege_football_players where year 'fr' but what if you also wanted to count a couple other conditions? Using the where clause only allows you to count one condition.
Try it out, see the answer, you can also define a number of outcomes in a case statement by including as many when/then statements as youd like: select player_name, weight, case when weight 250 then 'over 250' when weight 200 then '201-250' when weight 175. In the above example, the when/then statements will get evaluated in the order that theyre written. So if the value in the weight column of a given row is 300, it will produce a result of over 250. Heres what happens if the value in the weight column is 180, sql will do the following: Check to see if weight is greater than 250. 180 is not greater than 250, so move on to the next when/then.
Check to see if weight is greater than 200. 180 is not greater than 200, so move on to the next when/then. Check to see if weight is greater than 175. 180 is greater than 175, so record 175-200 in the weight_group column. While the above works, its really best practice to create statements that dont overlap. When weight 250 and when weight 200 overlap for every value greater than 250, which is a little confusing. A better way to write the above would be: select player_name, weight, case when weight 250 then 'over 250' when weight 200 and weight 250 then '201-250' when weight 175 and weight 200 then '176-200' else '175 or under' end as weight_group from llege_football_players. Write a query that includes players' names and a column that classifies them into four categories based on height.
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For any given row, if that conditional statement is true, the word yes gets printed in the column that we have named is_a_senior. In any row for which the conditional statement is false, nothing happens in that row, leaving a null value in the is_a_senior column. At the same time all this is happening, sql is retrieving and displaying all the values in the player_name and year columns. The above query makes it pretty easy to see whats happening because weve included the case statement along with the year column itself. You can check each list row to see whether year meets the condition year 'sr' and then see the result in the column generated using the case statement. But book what if you dont want null values in the is_a_senior column? The following query replaces those nulls with no: select player_name, year, case when year 'sr' then 'yes' else 'no' end as is_a_senior from llege_football_players. Write a query that includes a column that is flagged "yes" when a player is from California, and sort the results with those players first.
This table is pretty self-explanatory—one row per player, with columns that describe attributes for that player. Run this query to check out the raw data: select * from llege_football_players, the case statement is sqls way of handling if/then logic. The case statement is followed by at least one pair of when and then statements —sqls equivalent of IF/then in Excel. Because of this pairing, you might be tempted to call this sql case when, but case is the accepted term. Every case statement must end with the end statement. The else statement is optional, and provides a way to capture values not specified in the when/then statements. Case is easiest to understand in the context of an example: select player_name, year, case when year 'sr' then 'yes' doing else null end as is_a_senior from llege_football_players. In plain English, heres whats happening: The case statement checks each row to see if the conditional statement—year 'sr' is true.
Show source File line 91 def prepared_args @opts:prepared_args end prepared_modify_values the argument to supply to insert and update, which may use placeholders specified by prepared_args show source File line 102 def prepared_modify_values @opts:prepared_modify_values end prepared_sql returns the sql for the prepared statement, depending. Show source File line 133 def prepared_sql case prepared_type when :select, :all, :each Most common scenario, so listed first. Select_sql when :first clone limit 1).select_sql when :insert_select when :insert, :insert_pk when :update when :delete delete_sql else select_sql end end prepared_type the type of prepared statement, should be one of :select, :first, :insert, :update, or :delete show source File line 86 def prepared_type @opts:prepared_type. Show source File line 179 def run( block) case prepared_type when :select, :all all( block) when :each each( block) when :insert_select with_sql(prepared_sql).first when :first first when :insert, :update, :delete if opts:returning elsif prepared_type :delete delete else public_send(prepared_type, *prepared_modify_values) end when :insert_pk fetch_rows(prepared_sql)r return rst. This lesson is part of a full-length tutorial in using sql for Data Analysis. Check out the beginning. For the next few lessons, youll work with data on College football Players. This data was collected from espn on January 15, 2014 from the rosters listed on this page using a python scraper available here. In this particular lesson, youll stick to roster information.
Show source File line 172 def inspect prepared_spect " end literal_symbol_append (sql, v changes the values of symbols if they start with and prepared_args is present. If so, they are considered placeholders, and they are substituted using prepared_arg. Show source File line 156 def literal_symbol_append(sql, v) if @opts:bind_vars and match /A(. Z/.match s) s _sym if prepared_arg?(s) literal_append(sql, prepared_arg(s) else sql _s end else super end end log_sql whether to log the full sql query. By default, just the prepared statement name is generally logged on adapters that support native prepared statements. Show source File line 80 def log_sql @opts:log_sql end orig_dataset the dataset that created this prepared statement. Show source File line 96 def orig_dataset @opts:orig_dataset end prepare raise an golf error if attempting to call prepare on an already prepared statement. Show source File line 114 def prepare raise Error, "cannot prepare an already prepared statement" unless super end prepared_args the array/hash of bound variable placeholder names.
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Backbone of the prepared statement support. Grafts bind variable support into list datasets by hijacking literal and using placeholders. By default, emulates prepared statements and bind variables by taking the hash of bind variables and directly substituting them into the query, which works on all databases, as it is no different from using the dataset without bind variables. Public Instance methods call (bind_varsopts, block sets the prepared_args to the given hash and runs the prepared statement. Show source File line 108 def call(bind_varsopts, block) end columns send the columns to the original dataset, as calling it on the prepared statement can cause problems. Show source File line 121 def columns orig_lumns end delayed_evaluation_sql_append (sql, delay disallow use of delayed evaluations in prepared statements. Show source File line 126 def delay) raise Error, "delayed evaluations cannot be used in prepared statements" if @opts:no_delayed_evaluations super end inspect. Programmer friendly string showing this is a prepared statement, with the prepared, sql it represents (which in general won't have substituted variables).