how-to-Use-Columns-From-a-Subselect-in-Where-Clause-in-SQL

Author: samuelkaranja

sql-queries-12/how-to-Use-Columns-From-a-Subselect-in-Where-Clause-in-SQL/how-to-Use-Columns-From-a-Subselect-in-Where-Clause-in-SQL-mysql.sql

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+-- Scalar Subquery
+-- Find Students With the Highest GPA
+
+SELECT name, gpa
+FROM Student
+WHERE gpa = (
+    SELECT MAX(gpa)
+    FROM Student
+);
+
+
+-- Find Students Who Scored Above the Average GPA
+
+SELECT name, gpa
+FROM Student
+WHERE gpa > (
+    SELECT AVG(gpa)
+    FROM Student
+    WHERE gpa IS NOT NULL
+);
+
+
+-- IN Subquery
+-- Filter Students With Matching GPA of Non-Graduated Students
+
+SELECT name, gpa
+FROM Student
+WHERE gpa IN (
+    SELECT gpa
+    FROM Student
+    WHERE graduation_date IS NULL
+      AND gpa IS NOT NULL
+);
+
+
+-- EXISTS Subquery
+-- Find Students With Duplicate GPAs
+
+SELECT s1.name, s1.gpa
+FROM Student s1
+WHERE EXISTS (
+    SELECT 1
+    FROM Student s2
+    WHERE s2.gpa = s1.gpa
+      AND s2.id <> s1.id
+);
+
+
+-- Filtering With Multiple Columns From a Subquery
+
+SELECT name, gpa, graduation_date
+FROM Student
+WHERE (gpa, graduation_date) IN (
+    SELECT gpa, graduation_date
+    FROM Student
+    WHERE graduation_date IS NOT NULL
+    GROUP BY gpa, graduation_date
+    HAVING COUNT(*) > 1
+);
+

sql-queries-12/how-to-Use-Columns-From-a-Subselect-in-Where-Clause-in-SQL/how-to-Use-Columns-From-a-Subselect-in-Where-Clause-in-SQL-postgresql.sql

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+-- Scalar Subquery
+-- Find Students With the Highest GPA
+
+SELECT name, gpa
+FROM Student
+WHERE gpa = (
+    SELECT MAX(gpa)
+    FROM Student
+);
+
+
+-- Find Students Who Scored Above the Average GPA
+
+SELECT name, gpa
+FROM Student
+WHERE gpa > (
+    SELECT AVG(gpa)
+    FROM Student
+    WHERE gpa IS NOT NULL
+);
+
+
+-- IN Subquery
+-- Filter Students With Matching GPA of Non-Graduated Students
+
+SELECT name, gpa
+FROM Student
+WHERE gpa IN (
+    SELECT gpa
+    FROM Student
+    WHERE graduation_date IS NULL
+      AND gpa IS NOT NULL
+);
+
+
+-- EXISTS Subquery
+-- Find Students With Duplicate GPAs
+
+SELECT s1.name, s1.gpa
+FROM Student s1
+WHERE EXISTS (
+    SELECT 1
+    FROM Student s2
+    WHERE s2.gpa = s1.gpa
+      AND s2.id <> s1.id
+);
+
+
+-- Filtering With Multiple Columns From a Subquery
+
+SELECT name, gpa, graduation_date
+FROM Student
+WHERE (gpa, graduation_date) IN (
+    SELECT gpa, graduation_date
+    FROM Student
+    WHERE graduation_date IS NOT NULL
+    GROUP BY gpa, graduation_date
+    HAVING COUNT(*) > 1
+);
+

sql-queries-12/how-to-Use-Columns-From-a-Subselect-in-Where-Clause-in-SQL/how-to-Use-Columns-From-a-Subselect-in-Where-Clause-in-SQL-sqlserver.sql

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+-- Scalar Subquery
+-- Find Students With the Highest GPA
+
+SELECT name, gpa
+FROM Student
+WHERE gpa = (
+    SELECT MAX(gpa)
+    FROM Student
+);
+
+
+-- Find Students Who Scored Above the Average GPA
+
+SELECT name, gpa
+FROM Student
+WHERE gpa > (
+    SELECT AVG(gpa)
+    FROM Student
+    WHERE gpa IS NOT NULL
+);
+
+
+-- IN Subquery
+-- Filter Students With Matching GPA of Non-Graduated Students
+
+SELECT name, gpa
+FROM Student
+WHERE gpa IN (
+    SELECT gpa
+    FROM Student
+    WHERE graduation_date IS NULL
+      AND gpa IS NOT NULL
+);
+
+
+-- EXISTS Subquery
+-- Find Students With Duplicate GPAs
+
+SELECT s1.name, s1.gpa
+FROM Student s1
+WHERE EXISTS (
+    SELECT 1
+    FROM Student s2
+    WHERE s2.gpa = s1.gpa
+      AND s2.id <> s1.id
+);
+
+
+-- Filtering With Multiple Columns From a Subquery
+
+SELECT s1.name, s1.gpa, s1.graduation_date
+FROM Student s1
+WHERE EXISTS (
+    SELECT 1
+    FROM (
+        SELECT gpa, graduation_date
+        FROM Student
+        WHERE graduation_date IS NOT NULL
+        GROUP BY gpa, graduation_date
+        HAVING COUNT(*) > 1
+    ) AS repeated
+    WHERE s1.gpa = repeated.gpa
+      AND s1.graduation_date = repeated.graduation_date
+);